When translated, the names "Koitsu", "Soitsu", "Aitsu" and "Doitsu" mean "This Guy", "That Guy", "That Other Guy" and "Which Guy?" respectively. The names are whimsical and not meant to be taken literally. To keep the flavor, a more appropriate translation might be "Eeney", "Meeney", "Miney" and "Moe".
The "itsu" monsters appear as men of various solid colors flying on a paper airplane. Each is sitting in a different position upon its airplane. "Soitsu" stands in a heroic pose, "Koitsu" stands on one hand in an acrobatic position, "Doitsu" sits cross-legged eating noodles, and "Aitsu" looks away from the view point in a philosophical gaze. Each is one of the primary colors (except "Soitsu"), though "Doitsu" is arguably tan. Also, their Attributes reflect their respective skin colors: blue "Koitsu" is WATER, red "Aitsu" is FIRE, green "Soitsu" is WIND and yellow/brown "Doitsu" is EARTH. The four also appear on a few cards such as "A-Team: Trap Disposal Unit", "Fossil Excavation", "Fossil Dig", and "Gravity Collapse".
It is also helpful to defend them with traps such as "Threatening Roar" or "Scrap-Iron Scarecrow". "Machina Peacekeeper", "Roll Out!", and some other Union support will help. It is possible to use the fact that they are all Fairy-Type to your advantage, as well as their low individual stats, which make them all compatible with Limit Reverse. Since they are 4 monsters in all, this can also open up great emergency defense. Because each one of them is of different Attributes, if they're all on the field, the union monsters can be de-unioned to have all 4 attributes on the field for cards like "Fuh-Rin-Ka-Zan" and "Elemental Burst" for mass-destruction. Because they are only 4 monsters with no external support, it would be hard to base a Deck primarily around them.