Barri35, your statistics do change one thing for me: I'm even happier when a woman starts to shoot than a man, as it seems there are significantly more men than women who are at the range on any given day. My mother and grandmother taught me to shoot, so I always encourage women to participate.
Ruger GP100 are sometimes seen in a 3" barrel configuration. It is not as easy to find as a 4".
I'd recommend the 4" as the best 'blend' of lengths. I have a 3" .38 [rossi], a 4" GP100 and a 6" GP100. Firing the same .38special ammo in all three, the recoil is most noticeable in the 3" and least in the 6". A 4" will be short enough to do what you want it to, but long enough to dampen the recoil enough to keep it fun for a long time [as you said after shooting the S&W 686 4"]. Plus you get cool jets of flame with full-power .357mag ammo out of 4"!
A 3" will pack easier on the ATV, it will swing easier. It will jump more quickly in the hand also. So, do you want the extra portability of 1" less barrel length or do you want the extra control you will get with 1" more on the barrel length [plus higher velocities/more power from the same ammo]?
The age-old question: extra light for carry-often, shoot-seldom or a balance of weight for a lifetime of pleasure while shooting, while being light 'enough' to carry often [with the right holster/belt combo]? Only you can decide what is in your best interest.
I personally don't like the monogrips on the GP100, as the finger grooves don't line up 'just right' for me: whether it be the standard Hogue monogrip or the Hogue Tamer for the Alaskan. I have the 'old school' rubber/wood grips for the GP100 on both of mine. I found out [the hard way] that you can order either the full-sized GP100 rubber grip OR the compact-GP100 rubber grip [also fits the SP101, I believe].
For smaller hands and for carry, you may want the 4" GP100 but with the small rubber/wood grips installed, so the butt of the pistol isn't so big. The hand grip area of a SP101 with the strength and capacity of the GP100 = the best of both worlds?
I got my grips at MidwayUSA. I bought the wooden inserts for my full sized GP 100 from Altamont Grips. I like their GP100 inserts.