I cashed 4 years worth of tax returns when the IRS worker said I should. The IRS is now saying I shouldn't have cashed one of the checks from a previous year. Why??

Can't be sure. could one be a duplicate or perhaps the IRS issued a refund check for a closed year?  

It’s important to note that the claim for refund rules are extensive and complex with many exceptions, this is the general rule of IRC § 6511(a). Specifically, a claim for refund must be filed:

a) three years from the date the return was filed; or
b) two years from the date the tax was paid, whichever of such periods expires the later.
Filing a timely claim for refund under the general three-year/two-year rule isn’t the only requirement. IRC § 6511(b) then places limits on the amount of the refund by using two look-back periods.

(1) Taxpayers who file a claim within three years of the original return being filed will have their refund limited to the amounts paid within the period prior to the filing of the claim, equal to three years plus the period of any timely extension of time for filing the original return. IRC § 6511(b)(2)(A). 

(2) Taxpayers who don’t file a claim for refund within three years (1) above will have their refund limited to the amounts paid within the two years prior to the filing of the claim. IRC § 6511(b)(2)(B).

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if the above doesn't fit your situation and there was no explanation, call the IRS back to get one.