Deductible year of late safe harbor contributions

Small business has discovered that it did not contribute enough in the past 4 years.  They are making the correction and will make a contribution to make whole the contributions for the previous years. Can they deduct everything this year (the year the contribution is made for correction) or do they have to go back and amend the previous 3 years returns to show the deduction for the year it should have been made?  Any direction would be appreciated.

Well they can't deduct it if the contributions are made after the due date of the tax return for the year of the contribution.

And the 25% deduction limit overall.

Exactly - current tax year deduction subject current year limit.