do them with their backs facing the wall. then they can spider climb the rest of the way up. and be patient, in all my years i have never seen a 4 year old perform a perfectly straight arm back extension.
You are going to have to overcome a variety of "problems" that are packaged right along with all the "assets" that 4yo's bring to the gym with them. Three of these problems that I feel might be in play are strength, movement familiarity, and something that I'll call "muscle motion path" infamiliarity. Here's some thoughts on each one, nibble on them or just swallow whole in one bite.....
Strength.....It's definitely a wild card, kids can always improve their strength, but all kids are going to have a different starting point, as well as differing play time activities, and rates of muscle maturity. I've seen kids at 4yo come into the gym with a vast variety of healthy body types, all of them active, but not all of them strong, and some not old enough to realize noticeable increases in strength.......I think you can still get there, but you're going to have to dig deep into your bag of tricks.
Movement familiarity.......I can just barely remember experiencing "my" first straight arm roll as a teenager. It felt very different than a backward roll as it put so much more pressure on my my upper back and shoulder muscles, as well as a senstion of "stalling" on the way up. You maybe could set up a circuit with several skills/exercises, and spend your time at a station where you repeatedly lift them through the section of the roll where their hips begin to elevate as their weight transfers to their hands. You could skip the end of the roll, and make this a "round trip" motion to take them up to a "support phase" position-stall and roll back forward to prepare for the next "round trip".
Muscle motion path familiarity.......These kids may be training a lot of gymnastics, but haven't experienced this specific set of muscles being called upon to the task at hand. They may very well be strong enough to properly execute, but the specific set of muscles needed for the task are confused and don't know how to apply the strength they possess. Come up with a "very light" resistance exercise where they can move their arms in the proper movement repeatedly. Do not use more weight than you'd expect for a 20 repetiton set of exercises. They don't need to do all 20, they just need the weight to be light enough so they can move through the motion in a balanced, harmonious rythym.
After that, take two milk and cookie breaks, and call me in the morning.