Try to swap out any 3-creature habitat you have with the 4-creature ones in your storage. You do this by:
- Selling off one of your farms
- Placing the 4-creature habitat from storage
- Migrating suitble creatures from a 3-creature habitat
- Sell off the empty 3-creature habitat
- Move the new habitat to the space occupied by the old habitat
- Repeat from step 2 until you have run out of habitats to swap
- Buy a new farm to replace the one you sold
Of course you won’t be able to do this if all your habitats hold 4/5 creatures.