Can't tell you about this one as I've not worked on mine yet. Know about C2's. Yes you need to put wate in the tank for it to flush.
Go to the following thetford site www.thetford-europe.com English, customer service, user information, repair instructions, built in toilets, then select c4 you will then be taken to the pdf download page for that model.
or go to the Vehicles section sub section Caravans/motorhomes. [where your question should have been]
select page 17 from list at top of questions list look for 'THETFORD PORTA POTI BENCH TYPE WON'T FLUSH' In the answer page to this question is my email address [they don't allow adding it now] if you want a bit more of the do's and don'ts.
bob
May 2009