I wouldnt even crank a gas or glow engine on 4 acres, makes a good landing area for a beginner, if you have help from an instructor.
Unless you have 40- 50 clear acres to fly in adjacent to the 4, with a really good approach from at least two opposing directions, I wouldnt advise a 40 or 60 size trainer.
Folks dont realise how much area you really use when your up there.
Remember, these planes are mechanical and very subject to failure no matter who is flying and if you loose controll, you can KILL some one so, if there are homes or livestock where you are thinking of flying, do yourself a favor and forget it.

Happy flying