I'm new to these engines and don't understand the full function of the 4th
valve in this head.

Initially I thought it was an Aux. intake valve but notice that it is next
to the exhaust valve.

My Chilton manual doesn't provide any help..

May seem like a dumb question but it's got me stumped. Once I have an
understanding of this question, I'm sure I'm going to have a few more before
I get this nice little car ('85 CRX) running properly.

Thanks,

Steve