for jhin legend, if you cant channel 4 runes do you float 4 power?

No — you don't float power from a failed channel.

Channeling only places runes from your Rune Deck onto the board (Rule 430.1). If you're already at 12 runes, the "channel 4 runes" instruction simply fails — it doesn't convert into floating energy/power, and it doesn't ready existing runes either. The rest of Virtuoso's trigger (putting the banished spells in trash, drawing 1) still happens, but the channel portion does nothing (FAQ #10972).