I wish my cell phone and plan was that fast. For work we use an at&t plan, even with no wifi it is fast. But my personal phone through bo??? mobile the same thing would take 15 seconds, a guess and it is 4g.
So have you actually tested this on other phones and plans.
To me 4 seconds in blazingly fast.
Everything online doesn't take place in one millisecond you know.
If stripe is slow you can't do anything to improve the speed. Stripe is handling the connection so there is nothing to do on your side to improve the speed of that request.
4 seconds is acceptable. If you are able to show some kind of nice loader it should not be a big problem. Faster is always better but if there is no faster connection there is not much you can do. If you entertain the user that your payment process is 'busy' than waiting 4 seconds on a checkout page is acceptable imo.