Relatively few dbx 4BX dynamic range expanders were ever sold, but these units are still available online used from sites such as eBay. I couldn't find the hex codes listed for for the one I bought recently, but the original remote was provided with the unit, so I was able to learn them with my Pronto Pro 9600. Most available 4BX units are missing their remotes, so I'm passing along the hex codes for anyone who might need them.
Tested and verified working correctly in my Pronto Pro 9600
Power ON & Mute Toggle (NOTE: a 4BX always powers on muted, so enter this code twice into any system power-up macro): 0000 003A 0000 0008 0006 00B0 0006 00F9 0006 00B0 0006 00B0 0006 00B0 0006 00B0 0006 00B0 0006 01F8
Transition Level INCREASE (NOTE: the transition level sets the hinge point between upward and downward dynamic range expansion, i.e. which input levels are made quieter and which are made louder): 0000 003A 0000 0008 0006 00B0 0006 00B0 0006 00F9 0006 00B0 0006 00F9 0006 00B0 0006 00F9 0006 01F8