It sounds like you're looking for an existential quantifier. I'm guessing that TempRead is a sequence, so something like:
exists i in set inds temp & temp(i) > 400
If you literally mean "but not all of them" you would want an additional "and exists" to check that one existed that was < 400.
Incidentally, be careful combining two exists
expressions with an and
: you need to bracket the whole exists expression, otherwise the "and" clause is considered to be part of the first exists!