Rick,
I had a 4100 slightly up on 2 wheels when working on the side of an 18 degree slope. It was equipped with 3 - 60 pound rear wheel weights on each side when this occurred. I was using a mounted aerator at the time, but the mower deck was still on and secured in the full up position. The tractor is rather narrow and tall, both of which reduce sideslope stability. In addition, there is not much if any additional width gain by exchanging the rear wheels from one side to the other.
Many people have posted on this topic here, and you can review them if desired. My opinion is that you don't want to exceed 12-15 degrees to be comfortable. You have to leave some room for unexpected effects such as holes, bumps, and potential slope giveway.
The simplest and cheapest means to measure this is a carpenter's protractor level available for about $6.00 at home supply stores or tool stores. Place the level on a piece of plywood approximately 1 foot square to average out the local irregularities.
Most manufacturers do not publish a maximum slope capability but simply state that their machine meets a certain ANSI standard. Typically that means in a quasi-static tilt table test, all wheels of the bare (no attachment) machine stayed on the table up to at least 20 degrees. This does not guarantee that the unit can necessarily operate dynamically on any given 20 degree slope due to the numerous variables that are involved.
If you are in the process of creating the slopes around a new house, I would make a concerted effort to keep them less than 15 degrees if at all possible. If not, then mow the steeper parts with a walk-behind mower.
Good Luck!
JackIL