The 4400 is the earlier version, similar in design and function. The early 7015 models have the Deutz engine like the 4400, now they have the Kubota. The 7015 has several improvements and options. In my opinion the design of both of these is superior to other designs. The belt drive for the cutter wheel is more efficient than hydraulic and its easier and cheaper to repair. The engine does not tilt with the cutter wheel. The SP7015 with all wheel drive, remote control, Kubota 66hp engine and Razor cutting wheel is currently around $54K. I've had 3 Carltons, all performed very well. I'm looking at getting a new SP7015.
I have a 4400 with a blown Deutz. The rest of the machine is in good condition. PM me if you're interested.
Bill