https://larzm42.github.io/dom4inspector/ - game/mod content browser
http://www.illwinter.com/dom4/manual_modding.pdf - basic modding info
http://www.illwinter.com/dom4/manual_eventmodding.pdf - event modding info
http://www.desura.com/games/dominions-4-thrones-of-ascensions/forum/thread/modding-resources - this might be useful too

Folder
C:\Users\*YOURUSERNAME*\AppData\Roaming\Dominions4
contains mods folder which you need to overwrite with extraction of this mod archive[mega.nz]

.dm mod files are actually simple text files editable with notepad, but it might be good to get notepad++ and use this[mega.nz] dominions modding syntax highlight, though it is somewhat outdated and not complete

.tga files are editable with free GIMP program

Features:

• Units, magic sites, items, spells, events, nation - many of which are pushed to the limits
  
• Gunpowder rifleman unit, artillerist commander and sounds for them
  
• Ability to reach 140 level in all magic in under 10 turns
  
• Ability to reach maximum research in all magic in under 10 turns
  
• Ability to be stronger and better than everything and anything with just 1 province and no time spent
  
• Ready example of most modding possibilities
  
• Preach event to get huge population in province(works as mod nation)
  
• World enchantments unique to mod nation and triggering events
  
• Items unique to mod nation and impossible to drop except one harmful item

Ingame:

*Modding has quite easily reachable limits which are however not explained. Since some really OP items/events and pretender unit are pushed to their limits - they are a good indicator of those limits.
*Some pretty unique effects/things in original game can't be anyhow used in modding and most complex options are only available for spells.
*Sounds are weird. They play much faster in game than they actually are(twice higher sample rate of playback?). Thus mod sounds should be slowed down if they are to be normal. Included .sw files are raw(header-less) signed 16 bit PCM 22050hz sample rate sounds converted using Audacity program. You can actually import game sounds in Audacity using those settings or a-law for .al and they'll sound normally.
*Feldzeugmeister sprite taken from 'gunpowder units' mod on desura by user 'KateMicucci'.
*Global mod spells are nature, others(such as summoning) are blood spells. A few units are only summonable.
*Mod nation is EA, you can change it by changing #era command number. For instant ranged attacks(bullet shots) you can change(included as comment) #flyspr to -1. Instant ranged attack seems to happen before 'projectile' ones.
*Long sound(one minute) didn't seem to work.