Yes, a 4400 will handle a 643. My neighbor runs one on his. He had to put weights on the rear wheels, and has to run slower. We run a 5 row narrow head on ours that we built out of a 444, and a picker unit off a junked 2 row head off a 3300.
i would be carefull how much wieght you throw on the front of a 4400 we run a 213 flex head for everything we do and i think even that head is a bit heavy we had to put a fair bit of time and expense out two years ago and rebuild the front axle framing... it's holding for now but i'm more aware of how i use the header lift hydraulics -Mark in canada.
Where exactly did the axle framing breakIJ I am running a 1973 4400 with a 15ft flexhead, and have had no problems with anything breaking so far. I also run duals on my machine all the time. The rear axle is a different story. Mine has cracked in several places, and had to be welded.
well maybe it is our hills we farm lots of hills and perhaps there is more stress on the front end i also wonder if the newer flex heads are lighterIJ -Mark in canada.