...would I have to serve the 4/8yrs before I went to med school? Or could I immediately go to med school on a scholarship again and just add those years onto my undergrad?
#1 it depends. If you don't have the grades, premed curriculum, good MCAT score, and ECs you would likely have to complete your obligated service before medical school.
#2 it depends. If you have the grades, premed curriculum, good MCAT score, ECs, recommendation of PMS, Educational Delay granted, and a med school acceptance (or highly likely accepted) then you can immediately go to med school. Getting the follow on scholarships like HPSP are a separate application.
There are significant hurdles....good grades, time to do well in ROTC, medical ECs, ability to take classes outside your major as premed requirements......
A very small percent of students intending to go to medical school actually accomplish that goal. It's even more difficult if you have distractions like ROTC. Most active duty doctors recommend you do not do ROTC for your undergraduate degree to limit your commitment and to ensure you are able to fully commit the time for your premed commitments.
You are likely 17 years old and if you get the ROTC scholarship and do well in your undergraduate with an acceptance to med school paid for by the US Government you have now committed yourself until you are at least 30. UG 4years + 4 years med school + 3-6 years residency + 4 year ROTC payback + 3-5 year Med School payback.
It can be done but ROTC purpose for all services is to provide line officers (Army 16 primary branches).
Some of the hurdles my daughter experienced in AROTC prior to the Educational Delay were significant and nearly derailed her aspirations for medical school. Despite a chemistry major, not all the premed requirements are required for that degree. ROTC chose not to accommodate taking the extra classes in Human Phys for example (+others) because it was not required for her declared major. There was a direct conflict between mandatory ROTC classes and scheduling the premed requirements so guess what.... ROTC wins.
Go read the Student Doctor Network Forum. Don't give up your dream but plan well and plan for detours.