Terry Pounds's List: WPA Combined Connect

Yes, and no.

What you want to do is known as Multi-WAN or multi-homing, and it will allow you to "bond" two 4 Mbps lines into what is effectively an 8 Mbps line. However, there are several caveats:

 
• Without support from your ISP, you will not see 8 Mbps on downloads, only 4 Mbps. However, you can run two downloads, and each will run at 4 Mbps at the same time. The router can route new requests over whichever of the two lines has the most available bandwidth if you set up the load-balancer correctly.
 
• Most residential routers do not support multi-homing (at least with stock firmware). If you have a router which supports DD-WRT, I believe it is possible, but still very tricky to configure correctly. I'm not sure if a residential router can support it even with DD-WRT, given that they generally only have 1 WAN port and the rest are switched internally.
 
• Sticky connections are necessary for much of today's web, and might be difficult to set up depending on what software you're running on your router.
 

If this is something you want to play around with, I highly recommend putting several networking cards into an old computer that you're not using, and load up pfSense on it. pfSense offers something on par with most business-grade routers (very much like DD-WRT), but also has excellent multi-WAN support.