Both breast and formula fed babies usually move to 4hr E at around 4 months old. There are things to take into account on for both though, prem babies or low birth weight babies might not be ready for 4hr E until 5 or 6 months old. Although breast milk is really the best for baby, formulas are extremely well balanced.
All babies are individual of course. Mine was not early (13 days over) and within average weight range (smaller end of average) so would have been expected to get to 4hr E at 4 months but he couldn't manage a minute longer than 3hrs until he was 6 months and solids were introduced. Likely he couldn't take in enough in one go to keep him going 4 hrs as he had a small tummy, but he also had silent reflux so the smaller more frequent feeds might have made him feel better.