Your best bet is to concrete the gate post although it depends on if you have a concrete driveway? This will also impact which tools you should use.
The standard depth for installing fence posts and gate posts is a depth of 760mm (2'6") is recommended for a 6' high fence post, with 600mm (2') as a minimum, so this should be adequate for a 4' fence. Use either postfix or a mixture of ballast and cement to fix the gate post into the ground.
I've usually used a sledge hammer and pick to get through concrete and tarmac, then a spade for the rest. When you get obstructions a lump hammer and chisel or a crowbar can work when you're deep down and have no room to move. It's not very hi-tech but for one hole it'll do the trick! For concrete, Wickes do a good ready mix sack with the cement in a little seperate bag http://www.awin1.com/awclick.php?mid=15 ... nvt/220189
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