Hi Robert.
It's best to use a constant CSA: ie 4" or 6" all the way. Switching from one size to another may cause drafting problems.
In your drawing the water in the tank will cool the heat riser and may impair complete combustion - it would be best to site the tank some distance above the top of the heat riser to let full combustion take place before it goes near the cooling effect of the water tank. You
should also insulate the heat riser to promote a clean burn.
Looking again, I assume from the drawing that you propose to make the riser from metal so you can weld it into the tank? Before Satamax joins in "Metal Is Doomed"
It
may survive in your application if you stick to your original drawing design because the water would cool it down: however, I would strongly avoid cooling a riser - as mentioned earlier, your rocket won't burn cleanly.
In the drawing your tank is 'open vented' - it's using the pool as a sort of feed and expansion tank so it should be able to release any built up pressure safely. You may however get a flash steam effect in the water if the thermo syphoning doesn't work properly (which may squirt boiling
hot water into your pool). Use the largest diameter piping you can from the tank to the pool (and insulate the tank and the warm feed pipe) and ensure there is a sufficient gradient to help the syphoning. If you have to use a pump and it fails, you could get in a 'boom squish' situation - I'd try and rely on the thermo syphon. There's less things to go wrong without a pump.