Get a 240G! Just kidding.
Using troops for missions they're not specifically trained for w/unclear rules of engagement isn't new.
There are so many variables, we can probably only touch on some general guidelines.
If the attack is such that the firepower of the SAW will allow you to defeat it, or at least hold til reinforcements arrive, go with it.
If the M4 allows you to do the same with a reduced risk of civilian casualties, use it.
You are going to have to balance the nature of the attack (numbers, weapons, actions etc), the risk to civilians using the SAW, the greater risk to civilians/military personnel if you don't use the SAW and you are overrun. Some of the questions you ask can't really be accurately addressed without more information. All of this is balanced of course by the rules of engagement and your own common sense.
I would patrol w/a buddy in the environment you have described. I believe in working as a team where the sum of the parts "can be" greater than each individual working alone yadda yadda. If he's not up to speed, work with him. A buddy means a second set of eyes, another gun, a faster reload on the SAW, someone to drag you to cover and most importantly, someone to BS with since 99% of the time, it'll be quiet.