The statute of limitation only prevents additional tax assessments for 2004 tax year (assuming that the tax return was filled on time).
The statute of limitation doesn't prevent the IRS from auditing and making corrections - thus - yes the IRS may make adjustments on 2004 tax return.
Thus if the IRS determines that your client owes additional tax from 2004 - such taxes generally may not be collected.
If such corrections result additional tax for 2006 - the IRS is still within statute of limitation to assess such liability.
Sorry if you expected a different answer.