Are you trying to remove the whole 4" cover? Or just the small piece that holds the eyeball in place? Is there water over the return?
Any way to heat it up so the plastic expands and releases? I'm thinking blow dryer with a GFCI plug but only if the water is well below the return, you're outside the pool, no one else in the pool and you tie off the dryer so it cannot fall into the water. Also your hands, feet and ground are very dry. I realize that water and electricity do not mix so every precaution would need to be taken. My only other thought would be a blow torch but that, if not done with fineness would melt the return and/or burn a hole in the pool.
SAFETY is key. I have tried to think of every precaution but you know your situation. If you think it's worth a try, please be VERY careful. Take no chances.