A CRJ700 FLT CREW RETURNS TO LAND AFTER THE FO'S INSTRUMENTS MALFUNCTION. CAPT THEN CALLS IN FATIGUED AFTER FOUR LEGS ON THE FOURTH DAY OF A TRIP.

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IT WAS OUR 4TH LEG ON OUR 4TH DAY. WE RECEIVED ABNORMAL INDICATIONS ON OUR INSTS BEFORE TKOF IN ZZZ. WE COMPLIED WITH OUR MAINT PROCS WHICH DELAYED US 1/2 HR. AFTER TKOF AT 1000 FT AGL WE RECEIVED THE SAME PROB AND COMPLIED WITH OUR QRH PROCS AND CONTACTED OUR MAINT. THEY DETERMINED BEST COURSE OF ACTION WAS TO RETURN TO ZZZ TO SWITCH TO A DIFFERENT ACFT. WE RETURNED AND LANDED IN ZZZ WITH FO'S NAV. PFD INOP BECAUSE OF PROB AND QRH PROCS TO SELECT CAPT'S SIDE AS PRIMARY; RENDERING FO'S SIDE INOP. I AM STARTING TO FEEL TIRED AND LNDG AT MINIMUMS WITH HIS SIDE INOP MAKES THE APCH A BIT MORE STRESSFUL THAN NORMAL. WE ARRIVE AT GATE ONLY TO FIND OUT WE WILL BE KEEPING THE SAME PLANE AND MAINT FIXES OUR PLANE QUICKLY AND SIGNS US OFF TO GO BACK TO ORIGINAL DEST OF ZZZ1. I ASK FO IF HE IS OK; WITH FLYING BACK TO ZZZ1 ON OUR NOW 5TH. HE IS NEUTRAL; BUT WITH BLOODSHOT EYES. 2 PAX DO NOT WANT TO STAY ON AIRPLANE AND GET OFF BY THEMSELVES; SOME OF THE OTHERS ARE APPREHENSIVE TO STAY ON. I THEN CALL MY SUPVRS AND SAY; 'THE CREW IS FATIGUED.' THE FLT GETS CANCELED 15 MINS LATER.