I have a '95 F150 2 wheel drive regular cab LB with a 300 six and a 5 speed transmission; I've swapped in a 120 MPH Lightning speedometer cluster and it consistently reads about 4 mph faster than actual speed. I currently am running 235/75R-15s all around. My question is, should I consider buying a set of 17 inch Lightning wheels and 265/60R-17 tires(which is what I was planning on later) or should I swap my current tires for set of 255/70R-15s?

My future plans include finding a donor '94 or '95 Lightning and swapping the engine and suspension bits, money permitting....

 

Tires has nothing to do with the speedometer read out.
Tires will correct it if you are talking about speedometer vs GPS.

 

If the speedo consistently reads 4 mph fast I don't think there's anything you can do to correct it. Typically speedo calibration problems make it off by a percentage, not by a fixed amount. That can be fixed through calibration. If it's off by a fixed amount you can make adjustments that will make it accurate at one speed, but then it will be off if you are at any other speed.

If it's off by 4 mph at 60 mph and that's where you want it to be accurate, then you need to correct a 6.7% error (64/60=1.067, if you want to be accurate at a different speed, figure out the percent you are off at that speed). You could do that by changing to tires that are 6.7% taller. 235/75-15s are 28.9" tall, so 6.7% would be 30.8". 265/60-17 are 29.5" tall, or 2.2% taller. 255/70-15 are 29" tall, or 0.6% taller, so neither will correct it, but the 265/60-17 would be a little closer.

I'm not sure if / how the Lightning speedo affects things, but a '95 F-150 speedo can be recalibrated pretty easily. I don't know the procedure off the top of my head, but there are threads on here detailing it. I'd think even with the Lightning speedo you could do that. That's the typical way to correct a speedo rather than by changing tire size.