We must assign P, Q, R, and S to Monday–Thursday, one per day, while meeting all conditions.
Step 1: Apply restrictions on P.
P is not sold on Monday or Wednesday. Therefore, P can only be on Tuesday or Thursday.
Step 2: Apply restriction on S.
S is not sold on Thursday.
Step 3: Apply the Tuesday condition.
Exactly one of P or Q is sold on Tuesday. This gives two cases:
Case 1: P is on Tuesday.
Then Q is not on Tuesday. R must be before Q. S cannot be on Thursday. We check all placements under these constraints. This case produces exactly 1 valid schedule.
Case 2: Q is on Tuesday.
Then P is not on Tuesday. Since P can only go to Thursday (because Wednesday and Monday are forbidden), we assign P = Thursday. But S cannot be Thursday, so this is allowed. Now R must be before Q (Tuesday). Therefore, R must be on Monday. The only remaining product for Wednesday is S. This case also produces 1 valid schedule.
Step 4: Count valid schedules.
Each case yields exactly one valid arrangement, giving:
\[
\boxed{2 \text{ valid schedules}}
\]
Final Answer: \(\boxed{2}\)