Which is better: a blade with 3 or 4 teeth per inch for my machine?

This usually depends on the blade speed of your bandsaw (contained in your operator’s manual) and is expressed in linear feet per minute of operation. Saws with a rating of 4000 or less lfpm usually perform better with a blade with 4 teeth per inch.

Those blades usually have an overall length of 106 inches or less. Newer high-speed saws with a rating in excess of 4000 lfpm usually work best with 3 tpi blades that are more than 106 inches in length.