No, you will not get into trouble with the law for cutting the bills apart and spending the resultant $80. However, the set of 4 uncut $20 bills is almost certainly worth more than $80 to a collector, so you would be losing value if you did so.
moneyfactory.gov
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You can still buy uncut sheets from the Bureau of Engraving and Printing. This is not an error. Value is $4.
roughly $750.00
Eight dollars.
About 26 wide by 22 high
$40.00 There are too many uncut sheets out there to be worth any more than face value.
If you had a sheet of uncut two dollar bills, they would be worth far more than just two bills. Depending on their condition, the bills together would fetch between $2 and $10.
$32 would be the value. You can buy bills in sheets through the Bureau of Engraving and Printing at moneyfactory.com
Four genuine bills on a sheet will NOT have the same exact serial number.
Please check your bills again and post a new, separate question. No US $1 bills were dated 1987.
Like the value of anything, it is only worth what someone else will pay for it!