I think the Squarp Pyramid is a good non-4/4 sequencer.
it’s supports both polymeter and polyrythm and each track can have different time signatures.
for me, its probably the Pyramid’s USP.
(interestingly it was dropped from the Hapax, as most users found it too confusing for everyday use… a good demonstration of why non-4/4 is less common I think)
sure, its still has 16 pads, but thats really just arbitrary… you can use 7 or 23
(in the same way humans tend to default to 10 , due to fingers, hardware seq go for 16)
In the Eurorack world, I think the Frap Tools USTA is very interesting…
why? because it does away with the concept of a STEP = a fixed time unit.
so you can have five steps, were the timing is 1, 2, 1, 3, 4
(its quite interesting to read the USTA manual , and how this approach maps to musical notation!)
again, its got 16 hardware encoders, but again only convention, you can have 5 or 17 steps…
overall with hardware, there you have to choose a number of steps/pads/encoders…
and why not use 16? its not like 10, or 17 is any better…
(ok, @chapelierfou teletype is hardware… but it doesn’t have a UI so doesn’t count )
of course, on something like an iPad/VST, with only a virtual interface, that constraint is not there… but neither is the hands-on interaction
EDIT:
thinking about teletype, there are a few ‘hardware hybrids’ that make good sequencers, that are not bound by 4/4 conventions.
Monome Norms would be a good example in the standalone world.
and in the Eurorack world, I use the Percussa SSP, but you can also consider the ER301 or O&C.
all of these have the advantages of creative software , not bound by hardware controls/conventions, and give you a hardware solution, so something dedicated to task particularly with IO.
though by definition, also suffer the not quite-so hands on experience of software.
(*) there are many other interesting Eurorack sequencers… arguably eurorack is home of most creative sequencing, partly because you can play so freely with timing (as you can mess with clocks)
of course, things like VCVRack also have this same dynamic nature, so if you want creative non 4/4 sequencing in software, vcv is hard to beat!