What improvements/fixes has been added to this update?

Click on the link in the update and it will tell you.

Did You install it correctly, because I cannot...

Describe your problem in more details so that we could help you.

I getting loop when I try to install the HT205858 update, after I install update on OS X and computer restart, the update process didn't start. OS X start and I see once again message about update, the firmware in USB-C adapter is still old one.

I try multiple steps, reset SMC, PRAM, install combo 10.11.4 update once again, no luck...

I'm having the same issue. I download the update, install to the hard drive, click to restart my mac and then nothing. It just boots back into OS X without updating anything.

Same issue - usb keep going around in a loop

now for the last 2 days it is not asking to update, yet the firmware is still the old one...

After format and installation of clean OSX 10.11.4 update goes well. So anyone that have a problems reinstall OSX

"After format and installation of clean OSX 10.11.4 update goes well. So anyone that have a problems reinstall OSX"

That's not gonna happen to update a cable. My solution will be to take the cable to an Apple Store over lunch and let them just run the update. Never had a cable with updatable firmware.