The T4000 and T5000 series tractors are NH's "premium" line. Much nicer than the TD's, TT's and Workmasters. Either one will be a really good tractor. I have a 2007 model TN70A. It's been a great tractor, so far. Never had an issue. It's never been back to the dealer. Easy to operate and fuel effecient. The TN-A series was replaced with the T4000 series in 2008. (basically the same tractor)
You really need to see them, in person, side by side. The T5000 is considerably larger than the T4000. It's also heavier by 1200 pounds or more, depending on options like FWD or cab. The larger, heavier tractor will have more 3-pt hitch lift capacity, more FEL capacity, and will ride better than the T4000. On the other hand, the smaller tractor will be more nimble and easier to get into tight places.