You have 37MB (mega bits per second) on your FTTC/VDSL line. A bit is a 0 or 1. There are 8 bits in a byte or 11111111=1 of 8 bit byte, so divide by 8 and you get 4.625Mbytes per second. this is in line with the 4.5Mbytes you see reported.
Basically as others have said, you get the correct full speed, its just a different ratio of that speed you see. Bits for ADSL/FTTC connections and Bytes for cable/Ethernet connections.
8 bits = 1 bite