I found surface bond cement worked quite well, and was fairly easy for a first time user. It did not require leveling every block. Just level your ground and dry stack your blocks, and spread it on the outside/inside.

Quickrete, and sakrete both have a version that is available and Lowes and Home Depot.

http://www.quikrete.com/ProductLines/QuickwallSurfaceBondingCement.asp

http://www.sakrete.com/products/detail.cfm/prod_alias/Surface-Bonding-Cement

I would (did) use full size blocks 8X8X16 they are not that much bigger and you can grow lots of stuff in the blocks. I have strawberries, Garlic, and about every herb doing quite well.

There is a pretty big jump in stress adding the 4th row. I would discourage the 4th row (32"). Unless you have a level or solid footer, or plan on using mortar to help level it out. The flaws get magnified with every row, and for my dry stacked blocks the 4th row was really tough to get level. For my gardens the 32" garden has seen settling and winter/summer cracking. My 24" 3 row stack gardens have not.

As for reinforcing the walls maybe pour the corners solid with reinforced or fiber "crack resistant" cement. The ready mix cement is not the difficult to mix, just add water and mix until it is workable.

After completing my gardens I had to ask myself how much am I going to save my back by having high raised beds Vs. the gardening on the ground. I think in another 10 years I will break even. Though in my area the raised beds have worked wonders at keeping the bugs/ nematodes away.

Should I plan on lining the inside of the walls with heavy plastic to keep more moisture inside? A friend said watering their raised beds was a pain. I currently never water our garden, as it's in a low area near a creek that stays moist.

I've had good luck with 6mil black plastic sold in 8'x100' rolls at Lowes. It's lasted 10 years+ where 20yr landscape fabric has dissolved.