Well, I signed up for a free account and its videos (non-live) stream at around 188kB/s, which is obviously more than enough for a 4Mbps line. My money remains firmly fixed on your ISP shaping the protocols.
Funny thing about most of these speed tests is that they suck.
Let me try to illustrate using my line. My bandwidth is 600Mbps down and 100Mbps up.
Speedof.me:
Note that it also says it uses a Stockholm server.
BandwidthPlace:
It claims to have used the UK server since it's "the fastest." The Netherlands and German server fared far worse, despite being closer.
As for Iperf... I don't have the willpower in me to try and figure that out and the "http://dast.nlanr.net/Projects/Iperf/" URL is dead.
Here are the results of a local speed test provider called Bredbandskollen:
Here are the results from Speedtest.net to... let's say London again for teh lulz:
And Speedtest.net to "recommended":
The only two that reflect my actual bandwidth... You guessed it, Speedtest and Bredbandskollen.
Also... Jeez, BandwidthPlace and Speedof.me need to realise FIVEBAJILLIONMEGAPIXEL images don't make for easy viewing... Especially if all they're illustrating are three simple numbers (up, down and ping).