Actually you can!
Two ways:
1) Load balancing router (cisco has for around 200) - this balances at the connection level, and will distribute your traffic better, but not increase your speed.
2) Multi Path TCP - There are services starting to become available using this tech. Typically you buy a proprietary router, and sign up for a monthly service. There is some overhead, because it is typically a VPN implementation, but you will be able to combine services to get faster speeds. In your case you might get 3 or 3 1/2 Mb, but not 4.
I am using (2) with good success.