Yes they can be milked and inseminated like that. That's why you have fish farms and a hobby with tons of them.
You said you wanted natural. That won't happen naturally in a small tank. Unless you consider what happens in that video natural. Put simply the odds are not on your side if you want to sit back and do nothing and have pacu breed.
Most likely you will grow them out and eat whatever gets too big maybe some pairs to show signs of swimming together. Laying eggs and breeding will be a completely different matter. I'm all for you trying. If it works please share your experiences. But so far all your doing is dreaming about what ifs and arguing that you will be the first to accomplish something that farms dedicated to Fish farming have been unable to accomplish.
Best of luck in your endeavor. I promise you will not find anyone on this forum that will say you can breed naturally in a 400 gallon tank. Heck I'm one of the few who will say you can even raise them in a 400 gallon tank. My only suggestion is to work on the 4 you have...and wait until they reach sexual maturation before buying any more. Learn as much as you can about their behavior in that time and you will most likely reach the same conclusion as everyone else (if someone has bred pacu in a 400 gallon or laid eggs please correct me) which is breeding pacu in captivity doesn't happen naturally. Heck we can't even breed clown loaches naturally on a farm yet. And those reach sexual maturity at under a foot.
You are embarking into unknown territories without the proper knowledge (the internet cannot even help you) and without the proper equipment. Again I wish you the best of luck but you would waste less time, energy, and money if you went to your local fish market and asked them to supply you with pacu to eat. If you want replenishable source of food you need to find species that are able to reproduce in smaller settings...unless you want to put in the work to do what they did in the video.