I bought two of these 4ft x 4ft boxes to replace done wood ones I'd made, but had started rotting.

These are great because they are very easy to put together and unlike wood, won't rot. 

I've had my boxes for 3 years now and just replenish the soil each spring, and each year I have an excellent harvest.  I often plant tomatoes, jalapeños, garlic, cilantro, and cucumbers in these boxes.