Ibanez SR-300 4 String bass guitar re-fretIbanez 4-String Bassneck DR-300
Absolutely. There's no point at all in replacing something that doesn't need replacing. Unless you want a set of fatter frets or suchlike.
You've checked all the fretwires, and only the first four are worn/damaged. All the others are o.k. So just replace those four, which may well need linishing to match the others.
It's certainly possible to replace only some frets, and doing so is even common in some scenarios - for instance, on an instrument with Gibson-style binding where nubs of binding material are left at the end of each fret, it's a pretty major effort to replace all frets, so if only a few are damaged, it would be typical to only address those.
However, before you proceed, you may want to consider a few points. These concerns are somewhat subjective, if it's just a "player" you may not care versus if it's an important instrument that you want to preserve from an integrity perspective:
- Outside of some well known brands/guitars with well known fret specs, it can sometimes be harder than it sounds to get a good match on the profile and size of the old fretwire. Even when people use standard-sounding names for wire (i.e. "Jumbo" or "medium") those names mean very different things to different manufacturers in terms of size, shape, and sometimes even the alloy used.
- Depending on the condition of the other frets, even if you are able to match the wire, it may be difficult to get a completely un-noticable repair. At best, you may end up with nice shiny newly polished new frets, next to dull old frets. At worst, even with "matching" fretwire, you may need to do more work than usual to level and profile the new wire to match, especially if the old frets have been re-leveled in the past (which will give them less height and a different profile than when new).
When I approach a repair like this, I first consider if I can get a good match to the profile and dimensions of the wire. Some mainstream instruments (i.e. Fender, Gibson) have well known wire profiles, and even shorter versions of those profiles available to match existing frets that have been leveled. If I can get a good match, I consider the condition of the existing frets. If there's any question at all about their condition, I replace them all. Leveling new frets to match old ones usually involves "feathering" in to the old frets anyways, so in many cases it's not substantially less labor to leave them versus replacing them all.
You can get an idea on the wire specs needed by carefully measuring the existing frets. You can check width with calipers. Height can be a little more tricky, what I usually do is lay a straightedge across two adjacent frets, and then slip feeler gauges under the straightedge right next to one fret to measure the height of the straightedge above the fretboard. Unless you know your instrument is fretted with something non standard (i.e. stainless frets) you should aim for an 18% nickel alloy.
In the cases where I've only replaced some frets, I've always gone back over the existing frets and touched them up where needed, as well as polishing all frets from scratch so the sheen and brightness of the wire is all matched.