The "4th Dimension" is a modification for the expansion Forged Alliance for Supreme Commander which adds new content like completely new designed units, explosions and other effects. But this mod does not only add new content, it also changes parts of the game like balancing and gameplay. The goal of this mod is to make SC feel more realistic (as long as it is better for the gameplay) and to improve the balancing and optics of the game. For example nearly all muzzle velocities have been increased, and many units move much slower now. Another big aspect of this mod is the fact, that in 4th Dimension nearly all units have their right to exist, even in later tech levels, while in Supreme Commander you only see very few low tech units on the battlefield. Beside a new veteran system for vanilla Supreme Commander, which lets stronger units count more than weaker units and which gives units bigger buffs, the mod changes also the tarmac sizes and explosion effects.
Some minor balance issues on specific units. (Aeon T2 artillery shoots ~x5 faster than other teams T2 arty, Aeon T3 Battleship does ~1/3rd DPS of others, etc.) There's only a handful of these, and other than those the mod is un-freaking-believably good. Amazing done new units, new balancing of units actually does keep t1/2 units useful throughout the game, the textures are perfect, etc. Would be 10/10 if not for a small handful of overlooks in the unit stats.
Some mods just add units create by rescaling other ones, and are fond of "uber T4".
In 4th dim, the goal seems to be realistic units behaviours and characteristics (ships movement,etc...), balanced factions, graphical improvements... Making it the most impressive work done with SC.
Congratulations to the modder(s)!