EW1 12 valve SOHC ??

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.
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