Kitz, don't forget, the SNR displayed in the router wouldn't really vary at all. The SNR at 4Mb must have improved, therefore the router can now sync at 8Mb, thus bringing the SNR back down to pretty much the same as before.
J.Man, SNR on rate adaptive DSL is irrelevant when comparing stats below 8Mb (on normal ADSL). The whole idea is that the router syncs at the fastest possible value whilst maintaining the SNR at x dB.