If you have an auger, and it is 3pt mounted it will probably dig 4'. For a few holes, I would dig the rest with a manual digger. If you have many holes, buy or make an auger extension. You can make one from a peice of pipe and drill a few holes. With a 3pt digger and depending on how high its lifts, you may be able to use a 2-3 foot extension (3' extension is questionable). I have a loader mounted hydraulic digger and with an extension, I have gone as deep as 8'. With an extension, you will need to frequently raise and lower the bit to clean out the dirt from the hole.