The answer to your question is:
Yes, your IPSC experience will help. Unfortunately, against 4 BG's you are going to get shot. No matter how fast you can shoot, or how fast you can transition from target to target.
A better alternative would be to un-ass the area. Find cover. Do what you can to not get shot.
4 against 1 is a losing proposition.
IDPA does not 'require' slide-lock reloads. If the MD says it does, then (s)he is interpreting a rule incorrectly. Also, the only way you can be prevented from making up a shot is if the COF uses Limited Vickers. If the COF is regular Vickers, then feel free to shoot as many shots as you want.