Your solution depends on your resources to a large extent. If you have a high volume air compressor, or the building has air lines near by, you can reduce the 4" to a 1/2" or 3/4" air line and let it rip (just be sure the other end of the conduit is rigged to let water jet out and flow out.
No air, or the other end is in a finished room, then use something like this:
Run a garden hose on a stiff snake 20 or 30 feet into the conduit, at the lowest or the ends, prime the pump, and off you go. This will not dry the line, but you will bet most of the water out. From there, a long run with your shop vac on blow will move air through the conduit and dry it, though it may take a day or two to get it really dry.
Some try to put a rag of nerf ball or sponge on a line and pull it through, but if you do not know how the joints were made up and how many bends there are, you can get hung up very easily and create a solid obstruction.
At any rate, this has worked for me.