currently running 4 blade 30 pitch on a 42 twin step tiger what do i gain by using 5 blade props?

Depending on your current numbers, a more precise answer could be given. Typically, you will find advancements in your control and drivability. But the most notable increases are in your mid range cruise speeds.

We would need to know more to answer the generalized question.

The Tiger's we have worked with usually have the drive set too high and require a spacer. To start with we would need your performance numbers.
Top speed and rpm. Drive gear ratio. What motors do you have?
If you are having trouble getting on plane the 5 blades can help reduce the ventilation and in many cases reduce or eliminate the need to use trim tabs to get on plane. Expect to see an increase in cruise speed as well as improved handling and rough water capability.

Matt