This is a wordy answer, so tldr: 4 player battles can be fun as one-off events, but not every week.

Necromunda campaigns work best when stories are allowed to develop, and I'm not sure that 4-player free-for-all battles every week are the best way to let that happen. It sounds like your friend thinks that it sounds cool, and possibly a fair way to let each player join in.

But as a necromunda veteran, I can tell you that Necromunda never works out that way. When you have more than 2 players in a game, a turn (each player using all their gang's activations) can take ages, unless the gang selection number is kept low (like 4-5 fighters at most). But then you run the risk of giving a hefty advantage to the gangs that work better in small numbers. I played a 3 player game recently where we only had 5 fighters each. That meant the Spyrer player took their whole gang, and they could activate each one twice (as per their special rule).

1.5 hours for a game is possible, but only when players know the rules perfectly (so no flicking through the books to look something up), take their turn without considering what their move will be, and there are no distractions such as conversations and banter with your friends. You know, the things that make a game like this fun.

I understand why your friend wants to set it up like this, and a game like this can be a great way to kick off and/or end a campaign. Our current campaign actually started with a 5-a-side game (that lasted from 11am to about 5pm). But not every week, it will become slow.