if
[tex] \Sigma p_{partial} \ = \ p_{system \ , \ total} [/tex]
hence
[tex] p_{total} \ = \ p_{partial}1 \ + \ p_{partial}2 \ + ... + p_{partial}n [/tex]
... and you know p total , p1 , p2 , p3 (but not p4)
... and you know that those 4 gases are all there is ...
... what mathematical operation immediately comes into mind if you wish to calculated for the (unknown) p4 ?
regards
Ingo