Concrete base for retaining wall?

[froddan]

Hi!
I have a sloping yard and will attempt to build a retaining wall about 4 - 4.5 feet tall. There are some tree roots at the bottom, and I am considering laying a concrete base for the wall, with some rebar to make it a bit stronger. Then possibly using 17" blocks on top of the concrete.
It doesn't have to look good, just be strong and liable as noone will be able to see the "back of the wall".

Any advice is highly appreciated!!!