That is a great price also in 2021! Unfortunately, only the cable included with the PSB fits the M4U4 - there are none that fit from other manufacturers. A kind of mini MMCX. The sound is not neutral but with a lot of bass! But you will already know that.
I had this model back then and thought it was much better than the SE215 - in all disciplines
I'm sorry I'm late, I also have these PSBs, I like them a lot, they're not amazing, but they do the job well, at high volumes I think they do distort, so I listen to them at low, medium levels.
Regarding the cable, after several years, this is how they look, stiff and hard, I think the cable is the worst part of the product, it tends to fail due to the movement of the jack connected to the cable.
Try contacting PSB USA via email for cable replacement and their most indifferent response, something like:
"Please contact one of our authorized dealers."
I was looking for something similar without success, although I saw this manufacturer give cable spare parts for their iems that have to do with PSB, "MEE audio, pro6 cable"
This is the input plug for the cable to the iem:
The cable entry connections look similar.
AND.....
They work very well, I have not detected any significant change in the sound, very good for a new replacement and extend them a few more years.
I hope this post is useful for the few who have this iem.