As a former PRO IV owner ... I would reckon just about any replacement pickup (and removing the preamp -- and preferably incinerating it) will be an improvement. You will need to do a minimal* amount of woodwork to fit a standard Thunderbird pickup.
BTW, I'm not suggesting
all active electronics/pickups should be incinerated, just the ones in the PRO IV. To my ears they sounded a bit like wet cardboard. If you're into MM and active electronics, it's definitely worth checking out the EMG soapbars and preamps. Maybe
@Gaolee's Lace suggestion would be worth a look as well.
For passive pickups, at the cheap end, these work well:
Thunderbird Bass Pickup Vintage Style for Gibson,Chrome/Nickel
Or you can spend a bit more, eg these:
Link Removed
*minimal enough that when I saw how much had been removed and compared it to the price the luthier charged me, it would have been cheaper to buy a Dremel and do it myself.