I had a similar doubt once and my college coding club also wanted to organize a Div 4 so we contacted one of the problem setter of previous div 2 round. This was his reply:
"Afaik, there's no open way to propose a Div 3 contest on CF. There is a blog by KAN explaining the problem setting process for Div 2 and Div 1 rounds (even that isn't open to all either; you need someone who has achieved a master+ rank or has been a problem setter in a past contest). Note that you can still prepare problems on Polygon, host them in mashups (and conduct an event out of it), though those rounds wouldn't be rated. You might have seen The-Forces rounds, for instance, conducted this way.
Suppose you wish to propose a Div 2 round and are short of someone with access to the problem setting page. In that case, I'd recommend you first prepare a set of good problems (not necessarily on Polygon, just the basic problem and solution idea) and approach a CF coordinator (this is assuming that the problems you prepare are good) or you could find someone trustworthy who does, and ask them to propose the contest on your behalf.
In either case, there's plenty of fun stuff you can explore on Polygon that I'd recommend you to try out. You can check out this link for reference. Polygon has quite a learning curve; you'll be able to get to cool stuff quickly if you can get used to it beforehand."
I had a similar doubt once and my college coding club also wanted to organize a Div 4 so we contacted one of the problem setter of previous div 2 round. This was his reply:
"Afaik, there's no open way to propose a Div 3 contest on CF. There is a blog by KAN explaining the problem setting process for Div 2 and Div 1 rounds (even that isn't open to all either; you need someone who has achieved a master+ rank or has been a problem setter in a past contest). Note that you can still prepare problems on Polygon, host them in mashups (and conduct an event out of it), though those rounds wouldn't be rated. You might have seen The-Forces rounds, for instance, conducted this way.
Suppose you wish to propose a Div 2 round and are short of someone with access to the problem setting page. In that case, I'd recommend you first prepare a set of good problems (not necessarily on Polygon, just the basic problem and solution idea) and approach a CF coordinator (this is assuming that the problems you prepare are good) or you could find someone trustworthy who does, and ask them to propose the contest on your behalf.
In either case, there's plenty of fun stuff you can explore on Polygon that I'd recommend you to try out. You can check out this link for reference. Polygon has quite a learning curve; you'll be able to get to cool stuff quickly if you can get used to it beforehand."
I hope this comment clears all your doubt.