4 miles is a long damn swim. I was a competitive swimmer in high school, although not a distance swimmer particularly, and a "swim-a-thon" nearly ended my career. It was basically a two hour continuous swim for fundraising, after collecting pledges on a per-pool-length basis, so there was incentive to swim fast. I probably did about four miles in that, but strained my rotator cuff in the process, such that for several days I could barely lift my left arm over my head.
Anyway, if it turns out you really want to swim that far, you can find couch-to-1-mile swim programs on the internet that will have you swimming a mile after a couple months or so; once you get that far, you can probably just add a bit more yardage each week (~10% increase per week). However, I strongly recommend you look for some coaching. That's an awful long way to swim if your stroke is no good.