3-speed freewheels are for a 1/8 chain and are typically for a 114mm rear spacing. 4-speed freewheels are for a 3/32 chain (same as 5-8 speeds) and are also typically for a 114mm rear spacing. There is no such thing as a compact, ultra-spacing 4-speed freewheel.

As for the question, can you put 4-speed cogs on to a 3-speed freewheel body? For Italian freewheels like Regina and Caimi, no, you cannot. I can't say for other manufacturers.

As for putting a larger than 24 cog, probably yes, but depends entirely on the rear derailleur used for how much larger than 24 you can go. I have no clue how much larger your Simplex can go.