I purchased 4 Underground seas with some saving money when they were lower in value than they were now, and I'm wondering if i should sell them now or hold them in case of another price spike. There price has fallen steadily since it spiked and i don't want to miss my opportunity to sell while they are high. Any thoughts?

Which edition?
Also, you need to find buyers... don't expect to sell them as soon as you advertise them. The longer they remain for sale, the lesser its value will become.
If you think it is crashing again, I would personally hold.

They're not going to get reprinted, but even if they do the originals will at least retain their value. Magic is growing.

If you're not playing them, there's no need to hold on to illiquid assets. Dual Lands and the like are slow risers and if you're just holding on to them for speculation purposes there are much better places to have your money. Also by selling them you are helping to put cards back in to circulation where they will be played. So in a way it's better for both you and the community.

Im not hurting for money either, so im fine with holding them, i just don't want the value to crash