You need to decide how much you want to invest in the quaddie. If you want to spend $100 then try taking these number of horses in each leg.

2 x 2 x 7 x 7 = 196 units = $98 (for a 50c unit) or
2 x 3 x 6 x 6 = 216 units = $108 (for a 50c unit)

In other words, try and get 2 legs down to 2 or 3 chances then take as many as you can afford in the other 2 leg.
Good Punting.