I have a 3910 2wd and I have no problem lifting and carrying the same size haybale ovrszd showed. I have used my 6' Landpride 1X72 series bushhog on some serious hills no problem as well as a 7' Howse heavy mower. With the 7' on some steep hills the front end would get a little light but could have easily been solved with a few front end weights.
One summer I also ran a Ford 3400 with a 6' HD Bushhog brand cutter mowing steep roadbanks for the county. Never had an issue and that was the heaviest 6' mower I've seen.
I think your 4000 will be fine.