For the last 10 years I have worked primarily with 5 string violins. They now seem entirely normal to me, and I find looking at or playing a 4 string quite odd.
For many or most purposes I think a 5 string is a great option. The extra range is versatile, the low notes are exciting, and there's little reason to do anything else.
However in testing with a fast scottish fiddler I was recently forced to confront the issue that a five string may not entirely replace a four string under all circumstances.
A 5 string has extra pressure on the bridge meaning extra pressure on the top. It also requires a wider, heavier, and generally stronger bridge.
The upshot of that is that it takes more energy to get the violin moving because there is more weight and tension to overcome. Even though a five string bridge and top may be in a sense just as mobile as a four it is mobile at a higher energy level.
This effectively means that the 5 string, all else being equal, will be less quickly responsive at low energy input level. In other words when playing quietly, or (and this is the important bit) when playing the top strings fast and lightly, since they have less mass to drive the bridge and top.
So the response on the E string of a five string violin is slower than on a four. Many players like this as it smooths out the tone and helps avoid a raucous or shrill E string. However if you play fast reels, or Paganini, or anything else fast, high, and showy - you may find that you miss some lightness , sprightliness, or speed of response.
All this applies on an acoustic violin, and also on the Sycorax. On a regular rigid-body electric violin it is less relevant since the body doesn't respond much whatever you do at any speed. The sluggishness and deadness of tone of rigid body electrics is one of the main things I'm trying to overcome with the Sycorax, so it makes sense to pursue this to the limits of speed and light response with a four string.
If you play with a lot of bow pressure or a lot of effects this is unlikely to bother you. Metal players play high and fast, but I'm not sure if they will want 4 strings as a result or not since they are also likely to be playing harder and to want the extended range at the low end. But I can see how some classical or traditional folk players might value the lightest possible response over the extra range.
So this is a longwinded way of saying - the Sycorax violin will be available in a 4 string version eventually.
I plan to wait a few months before doing this, but it will happen.
If you are a player who wants a 4 string version get in touch - I will do it sooner if there is a specific customer who wants one!