You will not be doing an clinical rotation between MS1 and MS2... at that point you won't know a stethoscope from a tounge depressor. Generally AF HPSPers attend School of Aerospace Medicine in that break for a month. You CAN push back COT until this time and not do SAM at all. I have a friend who didn't do COT because he was in Brazil the summer before MS1 and did COT between 1 and 2 and won't be doing SAM at all.
Another important thing to remember is that COT is REQUIRED before you do anything else through the military. If you don't do COT you cannot go to SAM or do ADT rotations.
Advice time. Do not defer enrollment based on your AF responsibilities. The Air Force will make every attempt to match your schedule as long as it pertains to school. That is why I said that 'wanting to work' wasn't a good excuse to skip COT. If you turn down the 4-year and reapply for the 3-year you will not be looked at negatively. The AF needs doctors and will almost certainly accept you for a 3-year.
And this probably needs to be said to you. If you are not 100% OK with having someone else run your life then do NOT take the HPSP. Once you sign the papers, are commissioned and have the AF pay 1 cent of your education you belong to them for at least 12 years (4IARR in school, 4AD after residency, 4 IARR after that) where they can tell you what, when, where, and how and you have little input. I must stress that this scholarship is not for everyone and that if you are just looking for a way to pay for medical school then this is not it. There are a lot of flaws in military medicine and unless you are committed to being an officer in the Air Force who happens to be a doctor (meaning being an officer is 1st) then you need to reevaluate where you are. The match is going to suck and there is a chance you won't be able to do what you want when you graduate which is rare in civilian match.
In short, make sure you know what you are getting into and that you are committed to it.