In the second iteration the if-condition p{i} == [4;5] becomes true and terminates the loop. p{3} stays empty. In the second for-loop you want to acces that empty field --> p{3}(1). Matlab does not allow it and throws an error.
Hint: instead of
p{i} == [4; 5]
write
isequal(p{i}, [4;5])
or
all (p{i} == [4;5])
And if you post code the next time, please format it as code.
