As others have stated. Four ounces isn't a lot of weight. I put a 7oz camera on my 63" 40 sized trainer. It made no difference (to me) in how the plane flew. The weight might make a difference on a 3D plane in pulling vertical or not, but a trainer..... no sweat.

That being said, I'd rather find that four ounces by moving stuff around. If not, I wouldn't worry about it.

Plastic spinners are lighter (and cheaper) than aluminum ones.
Sometimes, depending on the location of the servo, the throttle servo can be changed to a micro and save a timy bit. That's only really worthwhile if you have one laying around.
Sometimes you can glue/velcro the battery inside the tail by using a dowel to stick it down there. Mine is located about 6" behind the wing just for that purpose.
Lighter nose wheel
I'm sure there's more tricks.