I'm using the mar-hyde high build primer for areas I fiberglassed. It's mixed 4:1:1, primer activator reducer. What can be used to thin it a little?

The recomended reducer listed on the can is what you should use.

I thought the reducer was to speed up or slow down the drying time.

Yes,

Kinda, it is used to thin the paint, and comes in different temps(speeds), but you MUST always use the same brand, and what is made for that specific paint, or at least what the manufacturer reccomends.

i dont believe marhyde has a reducer. in these cases, most of the time, a high quality urethane reducer can be used.
check with your local paint supplier.