A C172 CFI PERFORMS AN EVASIVE ACTION TURN CLB TO AVOID A PA28 IN THE TFC PATTERN AT SLN; KS.

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WHILE ON A STUDENT TRAINING FLT; WE APCHED SALINA ARPT FROM THE NE AND ENTERED THE PATTERN ON A R DOWNWIND LEG BEHIND 2 OTHER ACFT (C172'S) ALSO AS INSTRUCTED BY ATC. AS WE WERE NEAR ABEAM THE DEP END OF RWY 35 (THE ACTIVE RWY); A PA28 DEPARTED THE RWY TO REMAIN IN THE PATTERN. THE PA28 WAS ADVISED BY ATC THAT HE WAS #4 IN PATTERN -- WE WERE #3. THE PA28 RPTED HAVING US SIGHT. AS WE CONTINUED ON DOWNWIND; I CAUGHT SIGHT OF THE PA28 ON ITS XWIND LEG HEADING DIRECTLY AT US AND CLOSING IN FAST. I TOOK THE CTLS FROM MY STUDENT AND DID A HARD; CLBING L TURN TO AVOID THE PA28; THEN RESUMED ON THE DOWNWIND LEG I WAS ON. I RPTED TO TWR 'THAT PIPER TFC IS PRETTY CLOSE TO US.' TWR THEN ADVISED THE PA28 TO 'GIVE US A LITTLE SPACE.' THE FLT CONTINUED WITHOUT FURTHER CONFLICT. POSSIBLE FACTORS: THE PA28 MAY HAVE CONFUSED THE OTHER C172 IN FRONT OF US FOR BEING US. THE 3 C172'S HAVE EXACTLY THE SAME POINT SCHEME. LACK OF GOOD SCAN FOR TFC; BOTH THE PA28 AND US; POSSIBLE LOW WING VISION OBSTRUCTION; BUT NOT LIKELY SINCE ALTS WERE THE SAME. LACK OF THE PA28 CREW TO VERIFY THAT THEY WERE #4 FOR LNDG AND RPT HAVING ALL 3 ACFT IN FRONT OF HIM. POSSIBLE HIGH WING VISION OBSTRUCTION PREVENTING US FROM SEEING THE PA28 SOONER.