The YZ450 in any year model is a purpose-built MX bike, not an enduro machine. They all have high-strung engines (and the '03 is among the snappiest) and close ratio transmissions with a high low gear and a low top gear. The 4 speed isn't really an issue, but the 5's are a better choice. Low gear is the same in either one, and 4th in the 4 speed is about two thirds of the way between 4th and 5th on the 5 speed.
will help a lot. A heavier flywheel is almost mandatory. The engine can be pacified a little by using an alternative CDI. The suspension is a bit stiff for off-road, and the '03 can be especially tiring to ride for a long period until you get used to managing the engine. And you can forget running lights.
But they are very, very reliable, and a lot of fun in their own way. The '03 is a bit dated these days, but not bad. For your purposes, I think I'd pass on it and get a later model, or just something else. Try a test ride to see the difference.