I am running 4 and it's great.
I finally resigned myself to one class per account, and I do 3-4 characters of that same class on the account. This allows me to build multiple gear/skill sets that can be changed at the drop of a hat. Each class has a build for GRifts, TX Rifts and general speed farming. Also, Legacy of Nightmares opens up the door for a lot of testing on alternative builds. These alternate characters also serve as storage for gear related to that particular build.
1 class, multiple characters per account has drastically improved my quality of life in game while still offering the advantages of playing multiple classes.
4 accounts is super easy to do as well:
New Season > Use powerleveling community 1-70 on one account, asked that they T2 Kulle. Equip 4 piece Haedrig set.
Spend 10 minutes putting some other basic gear on.
Send bot out into Torment Rifts.
Come back 5 hours later, gear it up from stash. Rinse and repeat as needed until you can 4-man carry T6 rifts.
Once you can do T6 Rifts, then you add in your other 3 accounts using AutoFollow plugin and the AutoFollow "Follower PowerLevel" profile.
In about 3-4 Rifts all of your accounts are 70. Once that happens, Go do a T2 Kulle kill as a group.
Go back to town, equip all 4 piece sets you now have.
Drop any legendaries off of main account for follower accounts.
Spend a few minutes gearing the follower characters,
Change their skills to group buffs for sustain, DPS, de-buffs and crowd control.
Go back into Torment rifts changing to AutoFollow's "FollowerCombat" profile.
Come back in a few hours, gear up. Rinse. Repeat.
When you start getting multiple sets, repeat the T6 powerleveling process to make a new level 70 character.
TL

R I chose multi-botting for end game GRifting and class synergies. I got that, but I also realized a massive quality of life improvement across all parts of the game and botting.