Well, whatever the single-pip flag rank would have been called.
RDML... Rear Admiral, Lower Half
And there is no reason that commodores don't exist... They could have put a 5th pip on Picard (of the little line thing they used on DS9 for Deputy Director) to show that he still carries a senior rank.
Yeah, the fifth pip thing has been a fan speculation for some time. Frankly, it would've made more sense for a commodore to have the four pips boxed, on just the one side of the collar and with the pips still silver, to show he isn't an admiral. As I recall, all the admirals had boxed golden pips on both sides of their collars. -- RR