At Green Man Gaming if you use the voucher coupon code ELPRE-SIDEN-TE20P you will save 20% off the pre-order price of Tropico 4, which is already discounted 15% off. It would make the game only $27.20. The game uses Steamworks so it will require Steam.
In addition, there appears to be a new 20% off voucher coupon code: SUNNY-SAVER-20PEC
It might be good on their other pre-order deals, which are usually 15% off. I’m not sure when any of these deals expire.