It depends on your project. For something this tiny, the servo can drive the retract directly if it is cleverly set up. Basically so that the impact load on landing doesn't get transmitted into torque on the servo. For the mains, the retract direction is perpendicular to the direction of movement during landing and the balsa wheel pants hit the balsa on the wing and stop the gear from traveling further. For the nose gear, the retract is supported with multiple structures that prevent over-travel and is designed so that the loads during landing push the retract open and into these supports. For both the weight is supported by a laser-cut plywood structure and the servo just provides the torque. It's hard to see the support structure because it is so buried, but you can see it if you look. I would not recommend using this technique unless you are confident about being able to evaluate the loading and are willing to destructively test a servo to see what it can actually stand up to.
The picture in my previous post does not have power, so you can see how the structure and design of the retracts locks them extended under load.
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I think I've seen someone using this technique for a foam board FT P-51 on these forums, but I would not recommend using this kind of design for a 64mm EDF. I do put retracts on just about all my 64mm EDFs, though, using mechanical retracts like these:
Mini retractable landing gear for kt board rc airplane Sale - Banggood.com
So long as you can fit them into the wing and you make sure you are getting enough travel to lock the retracts, they are very light-weight, versatile, and reliable. Unfortunately, they have long lead times and are much more expensive than they should be.
Thanks for the reply. U just reminded me that I bought a several sets of mechanical retracts when E-flite made them for an F-15 many years ago. They might be too big for the F-4, but I did not know about the ones on Banggood. I will think I will order a few just to have and see what I can put them in. These R a little smaller than the E-flites.