I've had my 4400 for about 5 years and my 4320 for about a year. The 4400 is nearly identical to the 4300, though the engines aren't the same model power output is similar; dimensionally they are the same. I found though that I needed more lift capacity, more digging and wanted a "new" machine. I knew I wanted nothing to do with tier iv so that put the newest machines out. What I really wanted was the cab, but alas the tractor I bought was too good of a deal to pass up so another open station. What is it that you are looking to get out of your machine? I like the electric engaged pto much more than the manual lever on the 4400 it is a soft smooth start. Having the electric over hydraulic switch for the 3rd function is a big a huge improvement over a separate lever. I mostly use the 3rd function for a
grapple and it is very ergonomic to have the actuator right on the loader joystick.
Using both tractors all year long, I've found that the 4320 starts much quicker and less labored than the 4400. The new machine is quite a bit bigger and the operator station too reflects this. If you want the features of the 4020 series but the size of 4400 go with the 3020 series machine. A well optioned 3720 will have only a slight increase in capability over the 4400 but has the option of a factory cab. The 4320 is about 12" wider and 5" longer compared to the 3720/4400 though I don't find that there are any areas where I work that I can't access or maneuver through. There is a big difference in the weight however and I have to wait longer with wet soil unless I want to tear up grass on the slopes or leave imprints on the flats.
The one real knock I have against the 4400 was the thermistor problem. This thing would heat up and then prevent the fuel shutoff solenoid from withdrawing to start the tractor. It took me awhile to figure out that was the culprit, thought the fuel shutoff solenoid itself gave me fits. It would fill up with oil and then couldn't pull back to start the tractor. I'd have to pull the solenoid off and then depress the plunger and clean out all the oil. I wound up epoxying an o-ring to help. Anyway I hope all this helps.