when parsing the right-hand-side of the equals sign, you parse the tokens in order. first, you parse the 'x' token. it is 4. you remember that.
then you parse the '+' token, which is saved for the moment.
then you parse the ++x token. this says "increment x, then get that value". so x is 4, you increment it to 5, and return the 5. you now us the remembered value of 4, the +, and the value of 5 in your addition.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors