If you made $0 ROI on a $4000 spend, then honestly the issue may not completely fall on the traffic sources, perhaps it may lie in your targeting and tracking methods.
With PPC you can't just generally target, you have to laser target! You keywords must be very specific - and you have to monitor your campaigns or it will suck you dry.
Also, Adwords is not the only PPC engine, plus they have gotten very expensive in driving PPC costs through the roof! There are other PPC alternatives that you might want to try like Bing/Yahoo. I've tested them several times, and although they don't have nearly as much traffic as Adwords, they do have a dedicated stream of targeted traffic that may even out convert Adwords for you.
As far as paid adverting methods, I really haven't seen anything that gives you the speed and control of PPC advertising. You can speed it up, slow it down, pause traffic, or stop traffic complettely whenever you want to. I really don't care too much for PPC as a matter-a-fact tho. I'm more into social marketing which is more an investment in time than in money. But hey building relationships in business works!