^This
Been thinking about this for sometime now, and here are my arguments why the special set bonus should be on 4th piece, while the 5th piece still gives a normal bonus

1.More & Easier-to-achieve special set effects => more fun
It might suprise some pro players, but for the majority of players, we can't tell the difference of 100 spell damage, and most of the time, we don't care. The majority of players rather have some flashy and cool effects rather than "oh look another 100 spell damage niceeee". With 4th piece as the "special" bonus we can get more special bonus on a character and for most of us just the cosmetic "coolness" of set effects are more important than the stats.

2.More fun as new player/character to collect sets "randomly"
I donno how many ppl on forum still remember leveling up their first character, getting some gears from quests/bosses and put them on to see if it's good or not. Once you become more experienced, you know which sets are just better and you lose the interest of quest/boss gears you know are not good.
But for new players, they enjoy the process of getting some gears from a delves, quests, bosses or even from a treasure chest, but in current systems the sets require you to farm to get to the special bonus effects since it's hard to collect 5 piece of same set for just messing around. Reducing it to 4 makes it easier for new players to reach the special set bonus and improves their gameplay experience.

3.More build options
The most special set bonuses you can get is 2 (monster set) +2 (master weapons) +3 (those stat-only jewel sets) +5 (a full regular set), and it is hard for majority players to obtain master weapons, and the selection of 3-piece sets are quite minimal. Most ppl go with 2(monster set or master weapon) +5+5 (2 regular sets), and since master weapons are hard to obtain, normal players usually just end up with a monster set and 2 regular sets, the build options are actually quite limited (not as in "there is only so many set" but as in "we can only use 2+5+5 no wiggle room"). With 4th piece as special bonus, ppl can go 2+2+4+4, 4+4+4, 3+4+5, opens up a lot of options

4.Old build options still work anyway
If you don't wanna try out the new combinations, 2+5+5 still offers more bonuses stat wise than 4+4+4 or other combinations with 5 piece build up compare with similar 4 piece build up, since for big sets the bonus start at 2nd piece, and while keep using 5 piece build up will grant you the 5th piece normal bonus, 4 piece build up won't have that 1 more stat improve, so doing builds "the old way" will still work, even arguably better