Right. It updates once a week, usually on Wednesday. It went up from 2.64 to 4.35 this week.
The large majority of holdings in Muni MM funds seem to be tied to this index. Most rates seem to reset on Thursdays, with a bit of movement on Wednesday. I swapped yesterday, on Thursday.
I can confirm that I earned more than 4% on the 1 day yield yesterday in VMSXX by viewing accrued dividends by date.