Yes it is available in 4 ohm.
Two other things that come to mind about BMS products.
The mounting on the speakers is slightly different than most speakers. If you are going to buy a cone to replace another cone, check the diameter of the mounting holes, and also the hole it mounts in.
BMS diaphragms are not interchangeable from 8 to 16 ohms.
The gaps are slightly different. So if you have an 8 ohm driver, you cannot put in a 16 ohm diaphragm. And vice versa. I think that physically they will fit, but will not perform optimally.