According to Tony Sinclair :
My idea is to pour the concrete myself.

As others have mentioned:

100' of retention wall up to 4' high is a pretty massive undertaking.

Doing it a couple blocks at a time without proper design from
somehow who knows what they're doing just isn't going to work.
It'll probably fall over long before you complete it.

Your cheapest bet in terms of concrete is probably a poured concrete wall
rather than blocks.

4' of sand, especially where at least 2' of it is freshly disturbed,
_will_ push the wall over without careful attention to footings,
reinforcement and drainage control, regardless of whether you get freeze-thaw.

You're best off getting an engineer to design the wall first, and
ensure that you can get a permit for it.

Once you have an approved design, you can figure out what parts you
can do yourself, and what you need help for.

If you can do the forms (and probably rebar) yourself, then just bring
in the pre-mix trucks to fill it.

The biggest difficulty in doing it yourself (aside from exhaustion) is
the cost of material in the forms. It's quite possible you can find a
concrete contractor who has pre-made re-useable forms. In which case,
you're just renting them for a few days.

It may be possible to pour it in sections if you can reuse the forms.

You are _definately_ going to need professional assistance on this.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.