The government isn't exactly rushing to 4th doses. They could authorize it for under 50s right now, but there's little evidence that it makes a significant difference, so they haven't.

Also note that we still don't have vaccines for kids under 5.

If the government wanted to line the pockets of pharma more, there's not a lot stopping them. So it's hard to take blanket statements about this seriously. I'm not denying that this has happened in the past, just that it's a lot more complicated than any single conspiracy theory.