Any good .52-.56 4-stroke will work well in a .40 size trainer.

I've flown a trainer with a Magnum .52 4-stroke and it was a great combination. OS used to make a great .52 that was recently replaced with a .56. Saito also has a .56 and I believe Thunder Tiger has a .54.

All will work great for your airplane.

If it were more of an aerobatic plane, I'd say go with a .70 but it isn't necessary for a trainer.

I've heard people recommend staying away from 4-strokes on a first plane because a 4-stroke is more "complicated". I don't get that logic at all. They have the same two mixture screws as a 2-stroke and I've always found them to be extremely reliable and easy to operate. Of course 4-strokes do cost more and they often don't survive crashes as well. The torque and sweet sound is well worth the trade off though in my opinion.

Whatever you decide on, if this is your first plane, be sure to hook up with a local club and have someone there help you out. It will shorten the learning curve and make for a more enjoyable experience.

Here's an old trainer I had with the Magnum .52: