semi metallics
ceramics
organic
organic are softest but make the most dust mess and wear out the fastest, but are real easy on rotors,
semi metallics have metal flakes in them and so are harder of course and can take real abuse and last longer and less dust mess but wear rotors faster than organics, they resist heat alot better than organics, but they squeal often, on some vehicles like gm RWD cars caprices camaros k-10s etc
ceramics are deluxe highest price and resist temp increases and are hrdest i think and little dust mess if any at all
im not too learnt on the ceramics, so i dont know how hard these are on rotors or anything like that
but maybe someone here knows about them, uses them, or something
personally i like organics on light duty where i know i wont be hauling weight and such ever, to save on rotor wear, if i dont need to have semis,
and semis if i know i will be hauling weight and be using the brakes harder than usual and kinda often or more often than that
and ive never tried ceramics, i wont spend that much on something like that,
is just how i am, organics and semis work fine for me, whichever i need, depending on what i plan on doing with the vehicle
4 years on pads is a damned long ass time
you must either hardly ever drive, or there's some high $$$$$ pads/rotors on yours, or... youre running metal to metal right now, humm..
lol.
i replace pads and/or rotors at least three times a year on all mine, i drive about 3 or 4k, or so, each vehicle, each year,
new rotors and drums every second pad/shoe change is what you should do, everyone
good luck