How much salary should a senior java associate ask for working in BIG4?

Yoe?

You have 6 yoe with offer of 18 from PwC which is already 100%+ hike and 3x yoe, means it's already real good and above industry standards
For Big 4s it can maybe go upto around 24 fix for 6 yeo but as per my experience EY will just match PwC offer... Just ask for some joining bonus if they won't agree for more fix