A T4x ThinkPad with a supervisor password is a ticking time bomb. The password is not needed during boot and is only required to change certain BIOS settings, something which isn’t typically needed. But if CMOS settings are lost, the BIOS setup must be entered and the SVP will be required.
That’s exactly what happened to me. I had an old T42p (2.0 GHz Pentium M) with unknown SVP, happily working. Then somehow the CMOS got scrambled. I have no idea why, because the backup battery still seems fine. At any rate, the SVP was required and I didn’t know it. Bricked.
To recover the password, it can be read from an EEPROM but then has to be decoded. That may or may not work. Or a $100+ USB gadget can be procured—worthless for a single use because a replacement T42p system board would cost less. Or the EEPROM could be desoldered and replaced with a “good” password-less one (which I don’t have). Neither option seemed appealing so the T42p was sitting around gathering dust for a while.