I woudn't. Hipster style, but I can’t imagine much interest from companies or startups.
“4pay,” at least, is super short - that would be a different story. But two words with a letter in between just feels far too informal, imo.
I normally don't like numbers in place of words but this time I could visualize the 4 in a logo, I thought it would look good. With that said, I won't register it.