Arbusers

So I have found an handicap arb. So it comes down to this
I get a return of 2.12 with a handicap of +4.5 and a return of 2.08 on a handicap of -4.5.
But my question is. Do I always win if bet on both +4.5 and -4.5?
Or is it possible that none of the two are a winning bet?

Only in theory you are going to win whatever is the result.
You have to read both bookies rules because, in case of one player retirement, you
could have
1) win-lose (so regular gain)
2) win-void (extra gain)
3) lose-void (lost about half of your capital)

But if a player retires doesnt that mean that the other player wins automatically?

Not in betting. Usually my bookies void the bet and I suppose also yours.
Handicap/handicap is a quite sure bet, at 99% you will earn your small gain or, at least, you
get both bets voided if one of the player retires.
So forget my previous post but check rules out before placing the bets.
Moneyline and total games market in tennis is very risky for the upper matter.

Thanks, i will be careful but i am able to find many more of these opportunities but another question

A +4 and -5 is also an opportunity right?

No, -4 and +5 is an opportunity.

Well im kinda new. So i dont know.

How do you know wheter you an opportunity in handicaps?