so I just bought a 95 SC....

When the solenoid for the supercharger goes bad it will usually set a check engine light, tho it may not do it right away. On a 95 the code is p1640 (ask me how I know lol). When the supercharger solenoid (also known as a boost control solenoid(bcs) or a wastegate solenoid) goes bad you will usually experience a pretty significant drop in gas mileage. And depending on what position the boost bypass valve (bbv) is in when it does fail you could experience either boost down in the low rpm range where you don't want it (possible engine damage) or almost no boost at all. While not extremely expensive these can be very difficult to find (not many still available and if I recall they sold under a very weird part name) if you need help tracking one down let me know or if you're interested I have a used one for sale cheap. For more info on code 1640 read Nigelf's very informative writeup on the subject found under the series 1 scanning section. Hope that helps.

Scan tool says "quad driver 4 error" and I also tested my o2 sensors and I've got a lazy one...the one in the engine bay is slow to respond at idle but reacts fine at or above 2k rpm

Ahhh ok, quad driver 4 relates to the boost control solenoid, transmission ttc lockup solenoidor the evap purge solenoid. The majority of the time it appears its usually the bcs that goes bad. That write-up I reffered to gives detailed step by step instructions for diagnosing a code 1640. And just a word of warning, do no go with the Bosch o2 sensor. From what I've read on this site the bosch is too slow to respond and will give lazy readings. Denso seems to be the preferred choice of the board. Good luck!