During the first few years of WoW, you could easily play on a 512kbps line without experiencing any type op congestion (resulting in lag spikes). I started off playing on a 56kbps dial-up modem, and everything would be fine up to the point where more than 20 people appeared in my screen. My low bandwidth would become the bottleneck, and the latency would spike to 5-10k ms until I managed to run to a quiet area.
Later on, we used to play 2 people on a 384kbps line in a 40-person raid, without any latency at all. Even Alterac Valley with 80 people in it provided no noticeable latency increase.
But, I remember WoW got a new network setting at some point a couple of years ago, where you have the option of it spamming packets to the server at a much higher rate than was previously allowed, which increased the responsiveness of the game and reduced the latency slightly. Perhaps this would have killed a 512kbps line? This setting is enabled by default now. I can't remember what it is called, but if you dig around your WoW network settings, you should see it.
I know there was also a trick of playing with a windows registry entry to disable or modify a parameter in the TCP windowing algorithm, which could have improved latency but caused instability when downloading data. It had something to do with ACKing each TCP packet, as opposed to waiting for two or more before ACKing.
Lastly, there were services like Smoothping, that tunnelled all your WoW packets over SSH, which has precedence over normal internet traffic. This, in the past, gave a solid latency of just under 200ms where normally I got around 300-320ms - I have no idea how effective this is nowadays.
So, in short, you'll only have a better latency on a 4mbps line if your 512kbps line was congested, either by that "new" WoW network setting, or some background process in Windows causing crap, but typically the latency should be exactly the same.
