Can we apply for citizenship 90 days before the anniversary of 5 years of permenant residency? We can apply very next day when 4 years and 9 months gets over. correct me if I am wrong?
Yes, you can. You need to remember that 90 days is not 3 months.
For example, if you were approved 9/22/2004 you will not be able to apply 6/22/2004, as this is 92 days before GC approval date. The correct date to apply would be 6/24/2009.
That being said most people prefer to send their applications 85 days before the date on their green cards to avoid confusing a USCIS bureaucrat.