Love the MK4 3.5-10x40. Started with an M3, have the M1 now. They both have their own advantages, but I like the M1 better, overall. The M3s strong suite is quick adjustments for making hits on man-sized targets. The M1 is the way to go for finer precision.
Love the MK4 10X and 16X too. I traded off my 10X, for a second 16X, then traded that, for a 3.5-10 M1. After using the 16X for a few years, the 10X FOV seems huge. The only time I dial the power down, is when shooting through the chronograph, but that is easily done even at 16X.
The "real" MK4s (fixed power), go for about $800 on the used market, and the MK4 3.5-10x40 M1s I've seen from $550-750, depending on reticles, illumination and stuff. No way I'd pay full sticker price. With Leupold's warranty, and reptation for customer service, I'm happy to let someone else drive it off the lot, and I'll reap the savings.
A lot of guys crave the huge USO/NF stuff, and are willing to unload their lightly used Leupolds at bargain prices, so they can get what their friends have, and be a cool kid again.