We know that each large tree costs $4.50, and each small tree costs $2.25. If you buy 4 large trees, that means you will spend 4 * $4.50 = $<<4*4.5=18>>18 on large trees.
If you can spend a total of $50, then you have $50 - $18 = $<<50-18=32>>32 to spend on small trees.
The total cost of the small trees will be s * $2.25, where s is the number of small trees you buy.
To find the maximum number of small trees you can buy, we can set this expression equal to $32 and solve for s:
s * $2.25 = $32
s = $32 / $2.25
s = 14.22
Since you can't buy a fraction of a tree, the maximum number of small trees you can buy is 14.