anyone who keep farming the endless dungeon knows where farm the 4 slot wands (floors or sections)? until now i get a lot of crappy 2-3 slots and barely any 4 slots drops i think i get one 4 slots for each 50+ crappy wands.
Stuff I've noticed from farming in v1.09:
The only weapons I've ever seen with 4 slots are the blue flower wands, the rose wands (both of these have a fixed first passive), the wands with lots of pink petals and the wands that have a large green swirl around them. Getting 4 slots on one of those wands types is probably about a 15% chance.
I've never seen 4 slots on any other wand type, so I don't even bother picking them up anymore, except maybe the scythe sometimes just for the hell of it.
Getting further into the dungeon (up to floor 99) makes you more likely to get better stats on your wands. I haven't noticed it affecting the number of slots or the quality of the passives, though I can't read half of the passives so that doesn't help.
Common wands can drop from bags and chests. The rarer wand types can only drop from chests, whether they're chests from the rare sparkling enemies (definite), chests from the strong regular enemies like the yuki-onna (I think), or chests just lying on the ground (definite).
Every time you save in the bonus dungeon it halves your floor counter (e.g. if you save on floor 80 you'll go back to floor 40). If you're past floor 99 it'll take you back to floor 50 (the next floor you enter will be 51).
tl;dr - I don't think there's an easy solution to it. Just use a save that always starts at floor 51, focus on opening chests and ignore any wand type that isn't one of the four I mentioned above.
If you don't care about stats, you could try farming from rare sparkling enemies at the screen to the left of the second warp point in area 2. Just enter, check the sheep and the cotton fairy, and repeat until you get a sparkling one. I've definitely found 4 slot weapons from them before.