Yes I agree that 4 slots is getting a bit limiting now. It was mostly a decision I made when there were fewer Monster Girls, to not make it too repetitive, but now it's a good idea to expand on that a bit.
I'll probably reduce the playable area when you start the game, and then have some upgrades on the shop to buy more land in lots, and that will come with new slots available. Your idea about having Monsters around not generating anything is also interesting, I understand wanting more characters around. Thanks for the feedback!