Garage/outbuilding is 100 ft from house service panel is 4 gage aluminium wire overkill for a garage door opener and 6 lights and 6 recepticals?

A bit, but given the choices of available wire sizes, I think you will be fine.

#4 is already in a subpanel? what is the panel rating or are you just planning to run the feed to the garage. If not, how are you going to reduce the size of the #4 wire to a workable #-12/14AWG. Welders? 240V compressors? If you are only using 15 and 20 amp outlets, #4 is plenty big and may be overkill but it is always better to be too big than too small. #4 aluminum is probably cheaper than 6 or 8 copper.