At 4-7 days it is probably to late for you to PREVENT it, but you can learn by feeling the navel if an infection has begun.  By treating it vigorously then with injectable antibiotics you have a good chance to clear up the infection at the naval before the bacteria have a chance to get into the blood stream and spread to other parts of the body (like the joints).

Ask your source to make sure the navels are dipped soon after birth and the birthing area clean and well bedded etc.  That is where prevention is done.  But you will probably get a few infections even with those precautions so always be vigilant.  If you think a 4-7 day calf "might be infected but not sure", treat it to be safe.

In short order folks will be coming to you for advice on dealing with this problem!!

Re-read feelnrites "sermon"; it is very good. (On my beef calves I'm always looking for the "swing" too!)