Maybe I am mistaken but your 4120 is a recent aquisition is it not? The reason I went with the 110tlb and 4520 is the trailering aspect of it. Since I often have to transport the tractor and a couple of implements the combination of these combined (in footprint) can be a full load.
You may wan't to consider a 4720 with full weights and ballast too. R1 tires should put you near 6500 to 7000 lbs in a fully weighted tractor. I mention this because I would think you can reuse most of your implements this way.
I had a 4020 but not a 4520 in the past and it was alot of trouble to move around. Good tractors though, both of them. I have an 88 hp 3030LS at my disposal that is a cab tractor similar to a 2840, as well as the trucks and trailer to move it but honestly after several trips the 4x20 size equipment is just so much more convienient to use.