John,
I would not chance a 4120 on a 7K trailer. My neighbor has a 4320 and a FEL/BB combo and he hauls his (uneventfully, so far) on his 7K 18' trailer. He looks like Jed Clampett, though and I wonder how safe he is doing this. He also has a Ford F350 dually which stabilizes his load somewhat due to its size, but he is over the limit for that trailer based on weight. My 4520 with FEL, 72" BB, 4n1 bucket and loaded tires hit the scales at well over 6,000 pounds, and most 7K trailers weigh from 1200 to 2000 pounds, which puts one at--or over--the intended limit. These tractors are also too physicially large to be placed appropriately on that size trailer (as most of them are 16' or 18' long), which means the load is likely to not be distributed well. My advice would be to spring for at least a 10K trailer, if not a 14K trailer to ensure you have adequate axles and brakes to handle the load. Radial tires are nice and help also with towing confidience in this type situation. That said, if you are going to be doing mostly short, off highway trips with your machine, or tow very rarely, you MIGHT be able to get by with your current setup. No way, though, would it be my advice if you are doing much highway or longer distance towing with your machine.
John M
