When I first made my GameSpeed MOD I found it was impossible to make a mod that had more than 4 gamespeeds...Is this still true, or can I add a fifth gamespeed now?

You can add as many gamespeeds as you want. Only first four of them are visible in standard game snapshot screen, but other can be choosen via Custom Game Menu.

Thanks! I'll have to keep that in mind as I continue to develope my MOD...I am toying with adding a 1500 turn mode, but I wouldn't want to loose any of my 4 original modes.

You can add as many gamespeeds as you want. Only first four of them are visible in standard game snapshot screen, but other can be choosen via Custom Game Menu.
I've been trying to locate the file that defines how that snapshot screen is constructed (assumed it is done with python like other screens) so I could adjust the page and make new gamespeed options visible so I don't have to use custom game menu. Does anyone know where there that page is defined? Preferably I'd like to have room for 6 game speed options there or have scrollable box which displays 4 options at a time.