I believe the stipend comes once/month. For my DS, I think he received his first stipend in October and it was retroactive to September. I just depends on how quickly the unit processes everyone's paperwork and gets it submitted, then how quickly the government (I forget the acronym for the payroll agency) gets it processed. Also, as NROTC they are not ms4. That is an AROTC term. they are midshipmen 4th class or midn 4/c.
Pretty sure the payment comes at the beginning of the month too.