I'm looking into buying 4 tethrd skeletor climbing sticks, they're 20 inch's long. How high will that get me into a tree? Will it be high enough to bowhunt deer from a saddle.

Should put you in the 15 foot range, I'm sure that will be plenty high enough.

If you want higher a single 17" aider on each one will give you a significant increase in reach. The reality is we can't see the type of trees you'll be hunting from or cover provided from upbin them. 12 tob15 is plenty in many trees but in piney woods with little to no low cover 20 plus feet isn't necessarily a bad choice.

I use 4 Latitude speed sticks. They are 17", and I can easily get 18ft up. You should be able to get 18-20ft with yours, which is plenty high for any situation. You can also purchase a 2-3 step aider for the bottom step. That will definitely give you the option to get 20ft+ up.

I second the aider. Makes a huge difference. Was probably averaging 15’ from 4 hawk heliums. New and not super confident. I added one two step aider to the bottom one and easily get to 18’.

3 sticks with a single aider each and I'm averaging 16 feet to the platform. I'll probably pick up a fourth stick so 20 is more attainable.

I would look into one sticking, ultimate one stick. No cap on how high you want to, and can go. I will be switching next year. Plus can repel out of the tree I stress of having to climb back down with the sticks. Just bring one stick/platform assembly, instead of 3-4 sticks plus a platform.

I've watched some videos. A friend dies it. I'm not sure I'd survive it. If you are on public only, setting up and removing daily then for sure. But I hunt mostly private and it's a leave it up for a weekend or maybe move it a couple of times over a week or ten days.
But it's definitely an interesting way to do it.