It's many years too late to amend the 2014 tax return or to recharacterize the excess Roth IRA contribution to be a traditional IRA contribution instead. The only way to resolve the excess Roth IRA contribution is to be able to treat it as a Roth IRA contribution for some future year (perhaps 2016 or some later year if MAGI allowed) or to make a regular distribution of the excess now.
There is generally no statute of limitations on the penalty for an excess IRA contribution because Form 5329 is considered to be a separate tax return for this purpose and you would not have known to file this form to start the statute of limitations clock running. For each year that the the excess has been in the Roth IRA, beginning with 2014, Form 5329 must be filed to report the $4,000 excess and pay the 6%, $240 excess-contribution penalty. Assuming that there has been no year that the excess could have been applied, a $240 penalty will be owed for each year from 2014 through 2021 (a total of $1,920). The regular distribution in 2022 will resolve the excess so there will be no penalty for 2022 or beyond. Any earnings on the excess can remain in the account, so hopefully there has been good investment performance in the past and will be in the future to help make up for the penalty since those gains will be tax free once the Roth IRA is qualified by your husband reaching age 59½.
The Forms 5329 can be filed stand-alone since they have a signature block.
