I tend to find that although I plan to pack either horizontally or vertically it never ends up that way, though I've tried both in the past.
I now tend to pack in a manner of:
- How likely am I going to need that item.
- Will using this item remove it from my pack (food)
- Will using this item lighten my pack afterwards (gas canister / water bottles)
If I have something long and thin, then I try and keep it centred in my pack, working it in the same way my pack framework goes. Whilst if you find you have a lot of items like this you can pack them in in a manner which will leave your pack mostly vertical, but ultimately you will find that things won't stay this way (especially if like myself you get a little untidy, by the time I was done camping in Italy my pack was just stuffed full haphazardly for the plane home - this trip was a week long but we stayed in the one campsite).
As aucuparia stated, you will find you end up with a combination. What you need to aim for is a balance between weight and organisation. If I have two water bottles, one either side of my pack (usually externally but still could be internally) then emptying them evenly will help your pack. I tend to pack snacks in horizontally across the top of my pack, with cans and cooking equipment vertically (stacked) midway down. With clothes I like to pack either way as I don't notice a difference.
Try both out and see which is most comfortable for you.
Edit: since you stated it was for a 3-4 day trek, I'd recommend doing what I've done in the past which is a day long trek taking what you need for a 3-7 day trek and seeing how it goes. Do this a few times, weeks prior to your planned trek, this will allow you to see whether you prefer vertical or horizontal packing, and how comfortable your pack then is.