I've got a 4" corrugated pipe running down the side of my house, to
the street.
I want to connect my sump pump to this pipe, so the water discharge
runs to the street also. But my sump pump uses 2" PVC.
How do I connect 2" PVC to a 4" corrugated pipe?

You won't find any ready made fittings.

I recently did some work on our gutter drains, and saw adapters in our
local home center to go from 4" corrugated pipe to 4" rigid pipe. I saw
them at a few different stores, so I assume they are fairly common. You can
then use a regular PVC reducer bushing or two, or even a Fernco coupling to
get down to the 2" pipe. For that matter, a Fernco coupling might even fit
over the corrugated pipe? You'd have to try it at the store and see how it
fits. You might need a short length of pipe too to make the various
connections.

Four inch "drainage" pipe has a slightly different size then regular four
inch "DWV" pipe, but I'm sure you could find adapters to connect the two.

It would take a few fittings, but it should be doable.

Anthony