Hi Jan,
Thanks for your question.
Next month, I want to start putting all of my poker time in to PLO, and since it's been a little while since I played PLO seriously, I also plan to start at PLO25 and mix in some PLO50 after depositing. Therefore, as part of that, I did a lotttt of research on the best place to play PLO with the perfect winrate - a combination of many factors, not just direct rakeback.
If you're looking to 4table PLO25, I'd suggest William Hill.
As part of the ipoker network, which is characterized by big sports books, the network is generally fishy and of course, when it comes to omaha, fish have the mentality that 'I have 4 cards! How can I lose???' which is generally pretty easy to exploit.
In addition to a great 200% up to $2000 first deposit bonus which adds a decent slice of direct rakeback whilst you're clearing the bonus, you'll also have the William Hill VIP system. Raking $1000/month there will net you something like an additional 38% in rakeback (although it's hard to be completely precise, as this depends on player loyalty, like, the longer you play there and maintain a VIP level, the better it gets).
I can also tell you that, from railing a good friend of mine at PLO100 on ipoker recently, that the players are generally speaking, terrible. If you offered me 100% rakeback on PokerStars or 0% rakeback on William Hill, I'd play on William Hill for sure*, because the additional fishiness of players on ipoker far outweigh the rakeback elements.
*assume I was actually planning to play poker, not just open 690000 tables with 100% rb and fold every hand
Hope that helps!
-Marc