I am not so sure that the show would have actually worked in its fifth season. One problem of the fourth season was that the NX-01 stayed in Earth's neighbourhood and did not do any deep space exploration.
As much as I would have loved to see the Romulan War, it would have been tricky to handle. They would have had to either make a it a war of four species vs. one or to show how the Romulan succeeded in creating tension among the four species, attacked humankind and how the other three species only assisted them later. It can hardly be fought in deep space so how do the Romulans come so close to the future Federation core territory? Can you show an epic war in an interesting way while just two warp 5 vessels are in service?
The show would have had to ask some hard question and change much more than in the fourth season.
What I would have done differently? Not do the third season precisely because the Romulan War is already a long story-arc in the last two or three season.