To infringe a method claim, you need to carry out all the steps outlined for the method. Carrying out only 4 out of 5 steps should not qualify as infringement (unless you carried out some other fifth step that was considered equivalent to their fifth step, of something on that order).
Whether you'd be able to patent or not would depend. The patent wouldn't necessary be prior art that would automatically prevent patenting your 4-step method. At the same time, I can see where the PTO might consider it fairly obvious, especially if there were any mention of the fifth step being optional, or that you could get a useful result after the fourth step.
Infringing a patent requires that you do everything (or produce an apparatus that can do everything, etc.) in at least one independent claim of the patent. Despite that, if the patent specification mentions something, it can act as prior art that prevents later patenting, even if that something isn't directly embodied in any of the patent claims.