They're only off by about 1% and the bias depends on which side was up, it's not that bad. I wouldn't expect most inexpensive dice to be substantially fairer than that.
Honestly I'm sure this is the best solution. I get that a d4 is the obvious choice for something that should have a 1/4 chance of happening but a d8 with 4 numbers twice would be the most appropriate.
The only downside I can see is that a d8 and a d8/4 would be easy to mix up at first glance.
I’d actually like to see a d8/d4 hybrid. Basically take a caltrop d4, snip a bit off the ends to make a truncated tetrahedron. You’ll then have 4 large hexagonal faces and 4 small triangular ones. Put the numbers on the triangles. If it lands upside down, then it is just house rules whether to use the bottom face or to reroll. Or just number the large faces too.
It’s a similar concept to the round safety d4s; just less… round.