There are so many variable factors that go into this, that it's very difficult to give an EXACT answer to the question without writing a BOOK on the subject; but I'll do my best to give you general answers. Also; your English grammar is a bit confusing, so please forgive me for any misunderstanding on my part.
Strictly speaking; if there's a 50% chance that the ace is coming in the next four cards, that means that EACH of the next four hands has a 12.5% advantage, MINUS the inherent DISADVANTAGE for the 3 out of 4 times that the ace DOESN'T hit that hand, AND minus the disadvantage for all 4 hands when the ace hits the DEALER'S hand. It's YOUR duty to be able to quickly figure these odds for each individual situation; while AT the table. The best (and possibly ONLY) way to develop this skill is through experience, which is why El Hombre is SPOT ON with his suggestion that you keep perfect records of all your bets.
The first thing that you'll discover is that; in a real life situation, there is never going to be an EQUAL chance of the ace falling on each hand. For example; it might go something like this: The ace will fall on the first hand 20% of the time, on the second hand 15%, on the third hand 10%, fourth hand 5%,the dealer will get it maybe 5% of the time; and it won't show up at ALL 45% of the time. Again; depending upon the shuffle, your skill level, and other factors; YOU will have to make your best estimate of these odds at the time you make your bet.
I promised not to write a book, so I'll stop right here & cut to the chase:
In the situation you've described, I would advise you to bet no more than 6-8% of your BR PER spot. As your skill level goes up, you will be able to increase this somewhat.
In answer to your second question; if the ace doesn't come, the chance of the dealer getting it is usually LESS, as the chance of each subsequent hand getting it usually goes in descending order.
Feel free to ask any follow-up questions. SOMEONE on this board will be able to give satisfactory answers to anything on this subject. And don't forget SleightOf-Hands' advice: Take much of EHs' advice with a grain of salt.