Why? Isn't it personal preference? The stat increase from T4L4 to T5L1 is not very dramatic. The next upgrades are much more rewarding.
Okay. So I should upgrade from a t4 freshman to a t5 freshman, then upgrade that one to senior, then so on?
You can buy 2 T4 level 4 and a level 2 for 10b and get almost double or triple the stats. Go T4 young one
Plunder goes a lot more switching out t4 for t5. But plunder doesn't go up as much when you level up t5's. It's all personal preference but I'd rather get more money on my inactive farms than worry about my stats