Backing 4 horses per race it is hard to only take 50% of the market (unless you back roughies or it is a very open race) Your best chance would be if you leave out a short priced favourite. ie lay the fav as if you were a bookie only you don't take the whole field to beat it just your 4 picks.
If your strike rate is 63% you should on average bet less than 63% of the market to have any chance.

For example over 100 races you outlay 7500 (75/race average) but only 63 times do you get 100 back. Loss = 1200. However if you only outlay 5000 to get back 6300 you win!