A320 CAPT EXPRESSED CONCERN REGARDING ACFT CLRNC OFF RWY AS LNDG ACFT PASSED BEHIND AT PHL.

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WHILE ON APCH TO RWY 27R AT PHL; TWR REQUESTED THAT WE SLOW AS MUCH AS POSSIBLE AND S-TURN TO PROVIDE SPACING ON AN ACFT ON FINAL TO RWY 35. WE COMPLIED; AND WITH THE FO FLYING; LANDED NORMALLY. I KNEW THERE WAS ANOTHER ACFT CLOSE BEHIND US; SO I INSTRUCTED THE FO TO MAKE THE HIGH SPD TURNOFF AT TXWY K4. PRIOR TO TELLING HIM THAT; I OBSERVED A NUMBER OF ACFT LINED UP ON TXWY K; BUT IT APPEARED THAT THERE WAS SPACE CLRED FOR OUR EXIT FROM THE RWY. AS WE TURNED OFF; I OBSERVED AN ACFT ON TXWY K INTO THE INTXN OF TXWYS K AND K4. I STOPPED SHORT OF TXWY K WITH THE TAIL OVER THE HOLD SHORT LINE. TWR ASKED IF WE WERE CLR OF THE RWY AND MY FO IMMEDIATELY ANSWERED 'AFFIRMATIVE.' BECAUSE OF CONTINUOUS TALK ON THE FREQ I COULD NOT ADVISE TWR THAT WE WERE NOT CLR IN THAT POS; A B737 ROLLED OUT BEHIND US ON THE RWY. SHORTLY THEREAFTER WE TAXIED CLR. BECAUSE THE ACFT LNDG BEHIND US WAS A B737; THERE WAS NO PROB. IF IT HAD BEEN A WDB THERE MIGHT HAVE BEEN! THE MAIN CAUSE OF THE PROB; LACK OF PROPER SEPARATION IN TFC PATTERN. THIS ALSO CAUSED COM FREQ CONGESTION THAT PREVENTED ME FROM COMMUNICATING WITH TWR IN A TIMELY MANNER. CONTRIBUTING FACTOR: VERY BRIGHT LOW SUN THAT PREVENTED ME FROM MAKING A GOOD DECISION ON RWY EXIT POINT. ALSO; IT WOULD HAVE HELPED IF TWR COULD HAVE ADVISED US OF THE INADVISABILITY OF USING TXWY K4.