One 400W bulb over a 4 foot tank is not a very good idea.
1st you do not need 400W for the depth of a 75, which I believe is about 21". 400W is primarily used for deep tanks...not to mention it's a huge waste of electricity.
2nd one bulb over a 4 foot tank will leave the middle of the tank very bright and the sides quite noticably dimmer.
3rd if it's a 400W ballast, you can not use a 250W bulb. Also check what kind of bulb he had. Chances are it's a low K
rated bulb (probably about 4K) and you would need 10K or higher for an aquarium.
As far as temp increase, you will definitely see an increase, but it depends on your set up and how much air movement you have between the water and fixture.