If you want the most bud the fastest possible just fill that 4x4 space with all the 1 litre single cola plants you can. Anything else is screwing around which you can do but I wouldn't suggest.
If you are afraid to run that many plants, then you can grow them bigger, in those 2 gallon pots, and add a month or more of veg and just wait longer to crop out. Top them a handfull of times and lollipop them a little, don't flower until they're all 1.5' tall and as bushy as you could get them. You don't have to trim a lot off the bottom to lollypop with a 600w light, it has decent penetration.
Both methods yield a pretty decent amount of smoke, the small plants do it in half the time though.
It's far far easier and better to do them all the same. They will then all grow and crop the same, run dry about the same time, need food about the same time, need the light around the same height, take the same time to finish etc. Start mixing them up and you can end up overwatering small ones or large ones or loosing track of what needs water when, or have to use a bunch of crap to stack smaller ones on to get them all close to the light, etc. Overwatering may not scare you but it's an easy mistake to make when you have a bunch of different shit going on and it will sure as shit lead to root rot which also attracts bugs. Things that may sound simple may lead to large trouble...
Just try different styles with each crop if you like.
My 2cents anyway
