The number of 4 letter words  that can be formed out of the letters of the word PASSPORT  is

$2P,2S,4$ different letters are there  in the given  word 

(1)using 4 different letters fromP,A,S,O,R,T in ${ }^6{P}_4=360$

(ii) using two identical pairs{PP,SS}the number of 4 letter words=$\frac{4!}{2!2!}=6$

(iii) using two identical and two different letters from remaining 5 letters 

{pp_,_} and{SS_,_} the number of 4 letter words that can be formed =$2\left({ }^5{C}_2\right)\frac{4!}{2!}=240$

All different are  360, 2 alike, 2 different are 240 and  2 alike, 2 alike are=6.

$\therefore \text{ Total number of 4 letter words }=360+240+6=606$