I’ve said this before and I stand by what I wrote: I take Sulu’s line to be the literal statement Niven meant when he wrote the episode. A century before TAS, humanity (and humanity only) fought four wars (not skirmishes, not defenses from raids, not minor battles) with the Kzinti. People are welcome to take Sulu’s line out of context to justify how it fits into the whole Star Trek canon (so that I don’t get mistakenly labeled as someone who has ‘no imagination’) but IMHO it’s not necessary.
Except Sulu said the last war was "two centuries ago". If it was merely "a century ago" then there would be little canon issue with the statement, as the Kzinti would give Archer and crew something to do after the Romulan War. We would even be speculating that the Kzinti were taking advantage of post-War Earth or feeling out the early Federation and were rightly trounced four times.