CrossFiring another card (like HD 4850 as an example)with the integrated HD 4200 is a waste IMO,because it barely will effect your gaming performance.
Hybrid CrossFire is mostly for power savings and doesn't have a great effect on gaming.
So its better to get 2 high-end cards (like 2 HD 4850's as an example) and CrossFire them instead of CrossFiring a high-end card with a low one like HD 4200