Regarding how to take the first step toward using a mod:
If you go up a level from the Space Empires IV Forum, at the top of the list of forums is one called:
Scenario/Mod Archive
Select this forum. Then, select the topic which is titled: Space Empires IV. Now, scroll down until you see a message whose author is:
Tampa_Gamer
Look for a message that says: Mod_Pack 1.01
This helpful individual has put a couple of mods together in one package called the Mod_Pack. Download this file and follow his instructions. This will give you at least three custom races which are quite fun to play against.
This is just a first step compared to the changes you can make to the game. But it's as far as I've gotten myself... If you have any other questions, hopefully one of the more experienced folks can provide more info.
(I apologize for not trying out some of the other great mods out there. But while I'm here anyways, let me give credit and thanks, by name! even now. Thanks to Mephisto, Tampa_Gamer and Daynarr!)