I admit, I had to search a bit too, but there's no need to register to vote for 4d as Game of the Month on that site.
If you however wish to rate the game you have to register.
Anyway as Whichondoyou already pointed out for the non registering part:
At the mainpart (centerpart) of the page, you've got two boxes (Under the title Multiplayer games... that's all we do).
First box is titled News. The second box is titled GOMT.
In that second box you see centered 'May winner: Runescape' followed by a list of 5 games.
The last line (underneath the 5th game) of that second box is:
'VOTE! Total Votes: 15951 Top 100'
Now just click on 'VOTE!'.
You'll then get a list of the games you can vote for and 4Dimensions should be pretty high in that last. So click on it and you've voted then for 4D as Game of the Month.
And while you're at it, don't forget to vote on the voting links on our own webpage either