Solution to 4.

1. (a->b)&(c->d) /.:(~dv~b)->(~cv~a)
2. a->b 1 simp
3. c->d 1 simp
4. |->~dv~b
5. | ~b->~a 2 trans
6. | ~d->~c 3 trans
7. |__~cv~a 4,5,6 dil
8. (~dv~b)->(~cv~a) 4-7 cp