There is one last thing you can try before replacing the formatter. Delete the 4250 printer from your computer, then connect the printer via usb to your computer. If the computer recognizes new hardware it should be called something like HP USB Upgrade. Install this manually, choose Printers>HP>then choose your printer driver. After installation you should be able to run the easy firmware utility.
If the computer does not recognize the printer as a "USB Upgrade" then a formatter change would be necessary.
I was the lead on a printer refurb line some time ago and have done firmware updates on hundreds if not thousands of the 42xx printers. When I got the inflate failure I seldom had to change formatters. Not being able to get into the preboot menu is a good sign you need a formatter. Hope this helps.