Before embarking on specifics, a word of advice: learn to love plateaus. When beginning climbing, we make drastic improvements seemingly every time we go out. As we improve, gains become more and more difficult. There will still be jumps in ability, but they will become more and more sporadic. Learn to love climbing for the movement and adventure, for the striving towards mastery.
With that being said, here are the things that I would focus on:
Unless you're particularly unfit, you generally don't need to focus too much on climbing specific fitness at this stage. That being said, core exercises and gentle hangboarding would generally be beneficial at around the stage it sounds like you're at.
Take an honest assessment of your abilities--what are your strengths and weaknesses? You say you regularly flash V4...does that include V4 in every possible style? Overhanging pinches / smeary slabs / dynamic problems / .... ? Indoors and outdoors, on plastic / granite / sandstone / limestone / ...? Train your weaknesses. Actively seek them out---look for your "vegetable problems" that you know you should really work on, not just your "dessert problems" that you can flash to show off. Try climbing in styles that challenge your habits: intentionally use uncomfortably high or low feet, climb as slowly and statically as possible, climb as fast and dynamic as you can, ...
Intentional practice. So you just flashed a V4. Congratulations! Now what? You can never climb that problem ever again and have your ego adequately inflated, or you can critically analyze your performance---was it a "lucky" send that was super-shaky and seconds away from falling the entire time? Did you climb it perfectly? Try climbing that problem 5 more times (with rest in between), striving to make each attempt better. Hone in on the tiniest micro-beta, key in to precisely what body positions make the problem work. A professional musician doesn't play through a piece of music once and then try to perform it in concert; a professional golfer spends hours upon hours at a driving range polishing their swing. Similarly, strive for mastery, not just "sending."
Get honest critical feedback. While this could be as formal as finding a coach, try starting with seeking out stronger climbers (or even comparably skilled climbers) and asking them to watch you climb a problem and then critique your movement. Similarly, try recording yourself climbing and looking at the video afterwards---you can often spot better ways to move and eliminate inefficiencies when you aren't in the middle of trying to perform these movements. Think of it like a professional athlete reviewing every play on the game tape.
Perhaps especially pertinent: try harder problems. One very, very loose rule of thumb is that for sport climbing, your redpoint/project grade should generally be about a number grade harder than your onsight grade. For example, if you are onsighting 11b, you should be redpointing 12b. If you're flashing V4, you should be redpointing V6. As the adage goes, "If you aren't falling, you aren't trying." If you watch almost any climbing video ever, it will invariably show the hero climber falling. Over. And over. And over. Hard problems are hard and we learn by falling slightly later each time. Try seeking out a short (<5 moves) sequence that feels severely difficult, but not impossible. Practice just this sequence for 20 minutes. If you manage to do the sequence in this amount of time, it was too easy and you need to set higher goals. Come back to this exact same sequence each week for a month. The goal with this drill is to learn what your limits really are and what climbing with true, maximal, 100%, hold-my-hernia-while-I-try-this intensity. See this clip on Limit Bouldering by Power Company Climbing for tips on this drill.
The existing answers are very good, but I have some tips that were not emphasized enough:
Train with friends. You need to be very mature to be able to visualize and correct your own problems by creating movements that feel hard for you. Forget about that. Exploit the fact that people are different, and thus have different preferences. You might be very good at crimps and terrible at pinches, so you'll definitely profit from bouldering with someone that's better at pinches than you. Don't count on correcting your own mistakes by opening problems with pinches intentionally, because psychology says you'll try exactly the ones you feel less difficulty on. If your gym has a MoonBoard, go after all V4s you find, and focus on the ones you can't flash. Doing this with a friend is always more fun, too!
Boulder extensively. At this stage, focus in increasing your onsight bouldering grade. Instead of going for a V6, send four or five V4s and one V5. After one or two months doing this, send three V4s and two V5s, and move on to warming up on one or two V4s and sending three or four V5s. At this point, move to V6s. I think focusing on bouldering is the best possible way to understand your weaknesses and strengths. If you're used to sending V5s, then you should be able to execute any move in a 5.12a/b route. I'm not saying you'll send it, but you'll definitely do all moves separately. You can try to send the route afterwards to see what happens: did you fall out of exhaustion? Incorporate resistance training into your training. Did you get scared? Fall more and try to change your mindset. Bouldering frequently assures you that failing because of movement complexity will rarely happen.
Train your head. When I was about to sent my first 5.12, my mindset really bothered me. It was a barrier I had created for myself, and that I thought I had to break. Nowadays I see how fit I was for that grade - in fact, after sending my first 5.12, I sent five others in two months, all of them within two or three tries... But the first one took me two months, trying it once a week. My mindset was wrong: 5.12 is nothing more than a harder 5.11. This is valid for all grades. There are no plateaus. There are no barriers. If you feel stuck at a grade, this will definitely corrupt your mindset and make you think there's something that needs to fundamentally change for you to be able to send that grade, but most of the time, there isn't. If you regularly send V6/7 you should be able to send any grade up to 5.13- or understand that you didn't fall on that route because something mysterious was missing. Perhaps you needed a little bit more stamina, and that's all. Everything else is fabricated by that little demon that says that "you're still not ready for this grade". Ignore him. Don't let him enter your head.
A good tip I've used a couple of times, do that V5 project your stuck on and get someone to video you doing it. Watch it back, you'll be surprised what you see. Often what you think your doing and what your actually doing are two very different things.
V4 is pretty good (I don't boulder a lot these days and focus more on route climbing) I've been climbing for 10 years and I don't do much above this myself.
People who I know who climb harder than this often do for one very good reason, they just climb more often than me. I'm busy I only climb once a week, people who climb better grades than me generally go 2-3 (or more) times a week.