Interesting topic, could be because I'm planning to buy the same lens.
I've had 55-250 IS and 70-210 USM (oldie), and while I liked the 55-250 IS, something was missing. The old 70-210 USM was a great lens. A bit soft on on the edges, but better IQ than 55-250 IS. I also didn't miss any shots without the IS. I just shot with the tool I had. I'm not ready to invest in 70-200 f4 IS yet, but if my hands start to shake for some reason, I might have to
With the experience that I've had with those lenses, I think a second hand non-IS f4 will be just right for me. I've heard it's as sharp as the IS version, and I do have a monopod
Btw. I would've shot the shots above a bit darker, grass wouldn't need to be so bright IMO. 1/120 would've been just fine I think.