The answer to this is a definite maybe. All USB ports supply 5v, with a max current on most tvs of 0.5 amps, in accord with the first version of the USB specs. This is not enough to run a Roku reliably. It may start up and work okay for a while but when some program or channel puts a higher demand on it the Roku will falter.
Some tvs however do supply the 1 amp needed, but the general recommendation is to plug the Roku into house power to avoid potential problems.
What type of cable do I use? I want to try this. Hdmi on between Ultra and the tv like always, but what between Ultra and tv for power?
You can not power the Roku Ultra from the Tv USB port, there is no such cable that will allow this anyways.