 Notice that these two statements are not equivalent in meaning, even though the standard theory treats “if p, q” as just another way of saying “p only if q”. While (iii) states a condition under which I buy Lambert a cello (presumably he first learns by using a borrowed one, or maybe he hires one), (iv) states a (necessary) condition of Lambert learning to play the instrument in the first place (there may be others too). Understood this way, the statements taken together leave poor old Lambert with no prospect of ever getting the cello from me. But if (iv) were just equivalent to (iii), combining the two statements would not lead to an impasse like this.