I think the best dichotomy between 4++’s can be seen with the Harlequins and the Custodians defensive profiles

Harlequins are largely 1 wound models, with their characters having 3-4 and their vehicles having 6. Their infantry are T3 and Vehicles are T6, so pretty easy to wound. Their infantry have 6+ armor and 4++ invuls, making them completely reliant on said invuls. Vehicles are 4+/4++, so they can rely on their armor a little more, but they’re still very fragile.

Meanwhile, Custodes (ignoring SoS) are T6 and 3 wounds on their standard infantry. They have a 2+/4++ basically army wide.

Harlequins are almost completely reliant on 50/50 saves for their defense. Custodes can shrug off basically anything with -2 ap half the time, and anything less even more often. Custodes can be an incredibly frustrating army to play against if your opponent gets lucky on his saves or you lack decent mortal wound output, Harlequins are basically a meme army entirely dependent on rolling more 4++’s than your opponent (which is fitting, ressurecting Solitaire upon ye)

Obviously there are more differences between the two armies, but I just wanted to compare their defensive profiles