ROTH Conversion and Pro Rata Rule

[Inspectorth]

My husband and I are considering converting our remaining 4 Traditional IRA accounts to Roth this year.  Two of those accounts are nondeductible accounts (earnings are taxable) and the other two are traditional deductible accounts.  We have converted several other IRA accounts to Roth years ago and also have other Roth IRA accounts that were established as Roth.  I am assuming that the pro rata rule will apply.  But does this rule apply only to the 4 accounts that we are considering converting or do I have to include the previously converted accounts and/or the accounts that were originally started and contributed to as Roth?