Firstly if you are on your own a four man tent is probably excessive. People buy small two man/trekking tents so that it is easy to carry them and one doesn't have this sort of problem.
If you are in a larger group then it is not necessary for one person to carry the entire tent. You can break it up into at least poles, fly and inner. Different people can take each section which they should be able to fit in their bags.
If you really need to pack it on the outside of the bag no solution is really ideal. The best bet is to attach it in the lower central section of the bag, basically over the bottom zip. This puts most of the weight on the hips and keeps it central. The bag will probably still feel more swingy when you turn as the center of mass is further from the center of your body, but this can't be helped.
If the tent bag is small enough you may be able to fit it in the lower compression straps over the bottom zip. This is the easiest way to attach it but not particularly secure. You'll probably want some extra webbing/cord to tie it on better. On your bag the side and lower compression straps and pole hooks all look like decent places to attach extra strapping.
If you can't attach it at the bottom you could also tie the tent vertically in the center of the bag or even on top. These are less good as they put more weight on your shoulders and putting on top also tends to restrict head movement (you can't look up). While the side compression strap may look a good place to put the tent try and avoid this as, as you mentioned you'll probably end up lopsided.
