Is it okay to be a cofounder with less equity?
Equity should be “fair” for cofounders — that’s the only way to do it. That doesn’t mean it should be equal.
In this case, you’ve joined late at the 5th cofounder. 3%-5% certainly could be fair — it’s 3–5x what you’d get as a VP, and it’s still a good fraction of what the other 4 have.
Weigh your time-weighted contributions fairly. That will lead to the right outcome.