Well, what's done is done. It's not the end of the world. That's a lot of livestock for a 7-8 week old tank. Maybe even for a mature 150 gal tank. And those tangs make BIG poop! Maybe the added bacteria in a bottle will help speed the maturation of filtration to handle the sudden increased load. Keep an eye on your ammonia and nitrates.
Read and learn a lot. No matter how trustworthy your LFS is, there will be a tendency to accede to whatever your wishes are, regardless of whether those wishes are good or bad for your aquarium, then solve whatever problems arise by selling you more stuff. Better idea is for you to learn and avoid the bad stuff to begin with. This hobby is expensive enough done right, but it can cost 10X more if things are not done right.
Just curious. What is your set-up, exactly? How many pounds live rock, what filtration system, do you have a sump, do you have a skimmer, do you use supplemental powerheads in the display, what are your lights, do you use tap water or RODI, etc? Your tank is just like my tank. See my sig? It's got all that info. It will help everyone here a lot in helping you if we knew your exact set-up.
4 tangs is not THAT bad. I've had 4 tangs for over a year now, and I got them just like you before iknew any better. Sure, two of these will outgrow my tank, someday. Make sure to feed those tangs vegetable based food as they herbivores. Things like dried seaweed, or Nori, or unflavored sushi wrap. Here's a pic of MY 4 tangs. I know, I know, I'm a bad boy, too, but what's done is done.