There were several improvements from the 3 to the 4 in addition to the expanded FOV, which the 5 again adds over the 4. The "Vector" upgrade that had to be purchased separately for the TIR3 came standard with the 4, plus the unit was so much smaller. I never used a TIR3, but I know someone who has one. They still use it, they bought it used from someone who'd upgraded to the 4.
Another nice thing about it--unlike a mouse, keyboard, HOTAS, or most any peripheral, other than plugging it in and setting it on the monitor there's no moving parts! Your unit just won't wear out from heavy use like a HOTAS can,
I'd earlier just written off upgrading to the 5 as a "don't need to" item, now it's moved into the "wish list".