When I proc Jhin Virtuoso Legend ability and I am at 12 runes do I get 4 floating? Or does it just ready 4?

As established by RiftJudge FAQ #9949 and RiftJudge FAQ #10084, the ability of the Jhin Legend, Virtuoso, cannot be used to exceed the maximum of 12 runes on the board.

Because you are already at the maximum of 12 runes:

1. The "Channel" effect fails: Channeling specifically refers to placing new runes from your deck onto the board (Rule 430.1). Since you are at the maximum, the instruction to "channel 4 runes" cannot be executed and fails.
2. No "Ready" effect: Channeling is not equivalent to readying runes that are already on the board. The ability does not instruct you to ready any runes, so nothing will ready.
3. No "Floating": There is no game mechanic that converts a failed channel effect into floating energy or power.

In summary, if you are at 12 runes, the channel portion of the Virtuoso ability will simply fail to execute, and you will not gain floating resources or ready existing runes.