Can't tell you what to do.
But, here is my experience, FWIW.
I had the means to buy a really nice 5 pretty quickly after they became available. It was a Smith BT5. Wish I still had that one. Anyhow, I went right after gigging with the 5, but kept a couple 4's I still had simply because they were (and are) great basses. Then, as time went on, I had a 6 and then flipped a few 5's; and its true that as the lower notes became more mainstream, the 4's started to gather dust. Which seemed a shame.
Anyhow, the thing was, I never learned to use the Hipshot. A lot of cats went right to the Hipshot and got good with it. I just sort of took the attitude that I didn't need it and it was somehow the lesser way to get the low notes.
Within the last year, and for no particular reason I can point to, I started messing with the Hipshot. For me, this has been a real revelation. I've gotten used to the various tuning solutions with it, and find it gives me 99% of what I need, and the other 1% isn't a big deal. Well, this has suddenly made my 4 string basses practically viable again. It was almost like getting a couple of new basses. Also, there are some of the cover tunes that lay much better on the detuned 4. So, while I can play, for example, Beat It or Slither on a 5 or 6 without re-tuning, I will actually de-tune any of those instruments for certain tunes. The Hipshot on the 4 makes that so much faster.
Anyhow, I will probably always have a 4, a 5, and a 6 for various reasons. But, I find the 4's completely useful and relevant, even with current cover material. So, if you like your 4, try it with the Hipshot for a while, and see if you still find it useful. I did.
But, here is my experience, FWIW.
I had the means to buy a really nice 5 pretty quickly after they became available. It was a Smith BT5. Wish I still had that one. Anyhow, I went right after gigging with the 5, but kept a couple 4's I still had simply because they were (and are) great basses. Then, as time went on, I had a 6 and then flipped a few 5's; and its true that as the lower notes became more mainstream, the 4's started to gather dust. Which seemed a shame.
Anyhow, the thing was, I never learned to use the Hipshot. A lot of cats went right to the Hipshot and got good with it. I just sort of took the attitude that I didn't need it and it was somehow the lesser way to get the low notes.
Within the last year, and for no particular reason I can point to, I started messing with the Hipshot. For me, this has been a real revelation. I've gotten used to the various tuning solutions with it, and find it gives me 99% of what I need, and the other 1% isn't a big deal. Well, this has suddenly made my 4 string basses practically viable again. It was almost like getting a couple of new basses. Also, there are some of the cover tunes that lay much better on the detuned 4. So, while I can play, for example, Beat It or Slither on a 5 or 6 without re-tuning, I will actually de-tune any of those instruments for certain tunes. The Hipshot on the 4 makes that so much faster.
Anyhow, I will probably always have a 4, a 5, and a 6 for various reasons. But, I find the 4's completely useful and relevant, even with current cover material. So, if you like your 4, try it with the Hipshot for a while, and see if you still find it useful. I did.