There is a formula I use, that has worked very well in all my projects to predict where my active members will be:
Its formula of 15 for a 1000.
That is if you have an average of 15 signups a day for a quality site (decent rebilling), you should expect about 1000 active members for your site.
So to have 4000, you need to get yourself about 60 signups a day.
FTVgirls.com, when it broke 10,000 members a year ago it was doing about 150 signups a day.
Its a good way to predict where your active members will fluctuate at, or a way to create a goal eg. you want 20,000 active members? then you need 300 signups a day to reach it.
I bet though, that this formula would have been off quite a bit 4 years ago during the adult internet boom haha. But we are talking about today.