Onlyb,
I had to agree with Bruce88123 about the trainer, but you said you are still on the trainer which is great. As far as a 4* goes... If you plan on using the OS 70 on a 4* it's going to be overpowering the 4-star 40 and almost under-power on the 4-star 60. It's in a kind of grey region for the 2 planes. If I was asked to pick I would say go with 60 size. Bigger planes are just more smooth over all. Like I said, the plane will be almost underpowered with a 70 but you'll still be able to fly it just fine. You just won't be able to do unlimited vertical!!

Hope this helps.

Ken