Is it normal for severe bruising to occur 4 days after a groin injury?

Bruises. Bruises can get darker as they heal. They can turn yellow, darker purple but should resolve in 2-3 weeks. Avoid non-steroidal antiiflammatories like advil, ibuprofen, aspirin.
Varies. The bruising is an indication of bleeding into the tissues involved in the injury. If the bleeding is deep, it will take longer for the"bruing" to become evident. Gravity further "pulls" the bleeding down lower than the injury. Bruising is more apparent if on "blood thinners" (e.g. Coumadin (warfarin)) or antiplatelet drugs (e.G aspirin, pletal, Plavix) or older (platelets not as effective).