Jennifer, do it gradually, you may find there is one feed he happily extends & others that are harder.
2 ways... extend each by 15mins every few days or work on extending 1-2 feeds & then the others... it can be a bit messy.
you don't need them all to be 4 hours & it never needs to be an exact thing either for some babies. Liam goes about 3.5 for most of his & one 4hour, because he goes a long stretch 9-10hours at night (second bub - angel & I am more clued up) so to get 5 feeds in I can't do 4 hourly. On the flip side, my niece 4mo, won't feed closer than 4.5 hourly & often 5 hours & so my SIL is stuck with a night feed (also second bub) because K will not feed in under 4.5 hour ever!