$4 CPM deals for pop-unders are rates premium online marketing companies charge these days for geo-targeted pop-unders. Actually, many would settle for about half of it.
I'm pretty suprised you charge a $4 CPM rate for what seems to be traffic from guaranteed hits services, which generally are not know for their quality of traffic.
For half of the price advertisers could be advertising on well-branded, very high-traffic and high-quality websites daily featured in online and offline publications, something I am not sure about many websites active in guaranteed traffic networks are.
Not trying to rain on your parade at all, but you might want to review your rates a bit if you'd like a ton of advertisers ...
Best,
Mike