hello TG,
as long as the bottom of the tank , ie where the pipe/tap exits the tank , is above the veggie plot , then there will be no problem of water flow. If the plot is higher than the tank outlet , then you would have to pump water up there . You could , in that case, use one of the old cast iron replicas of water pumps , for filling buckets etc .
If the tank IS higher than the plot and water does flow , then as long as it is for bucket /watering can filling , there will be no problem . If you want to use a hose with pressure , then you would need a fairly powerful pump .
Water will always flow down hill , not up hill. It will just be a slower flow than from your house tap, if you are on high pressure mains .
Also remember that water weighs about 101/4 lbs a gallon , so make sure that the tank is on a very solid , level base , ie breeze blocks laid flat , as a 400 gallon tank could weigh 4000 lbs plus the weight of the tank when full !!! , that is not far short of 2 ton , take care.
Hope that helps ...
Cheers
Russ