Confused with Internet (Comcast and Router)

I get my internet from Comcast. Its website says the download speed is 20 Mbps. Strangely though, I've achieved max Steam download speeds past 4 MBps. I have my router set at its highest setting, which is "up to 300 Mbps". I'm confused how I could achieve 4 MBps downloading in Steam when that is past the 20 Mbps Comcast says I get. Can somebody explain what exactly is going on and answer this question: would getting a better router improve my internet speed or is it already enough and I would have to pay Comcast more to get better internet speed?

 

You don't need a better router- 20mbps is roughly the max achievable speed on 802.11g (up to 54mbps).
Yours is 802.11n (up to 300mbps in theory and closer to 60-70mbps in reality) so your WiFi can do more that your ISP at this point.

You may end up getting max speed that is higher than the theoretical speed of your connection because ISPs often use "boost" allowing some data to be transfered faster and then slowing down to what you're paying for (mainly because that makes them look good in speedtests).

So ignore max speed and see what's your average speed on big downloads.

 

That makes sense. So you're saying that there's nothing a new router will improve because of my internet plan.