After starting on 4, I bought my first 5 when I graduated high school. Before I started dating the woman who became my wife, I hadn't owned a 4 string since '98. She been playin bass for 5 years and is quite content slamming away on a P bass (despite trying to convince her to try workin on a 5) and I'll play it every now and then and have used it at church a few times. Would I miss having a 4 available? Not in the slightest. Unless my couple buds that I write music with wanted to do stuff in like dropped C or some other in between tuning.
All this being said, I'd say keep it. It doesn't take up a large footbprint in the closet and if you sold it for cheap, you'd kick yourself after a week.
All this being said, I'd say keep it. It doesn't take up a large footbprint in the closet and if you sold it for cheap, you'd kick yourself after a week.