Account balance plan with earning and then FICA taxed

A participant defers a portion of his salary in January until termination, and each month the deferral account is credited with 4% interest.  When FICA tax is paid at the end of the year (using the rule of convenience), does the amount subject to FICA include the original amount of salary deferred plus the interest accrued throughout the year, or is the amount subject to FICA only the original amount of salary deferred? 

I think you need to include the interest accrued throughout the year. See Example 2 in Treasury Reg. 31.3121(v)(2)-1(c)(4), which seems to be close to your fact pattern. 

Agreed, the amount subject to FICA is the benefit accrued (balance) up to the point FICA is applied, but then any incremental interest thereafter is not subject to FICA.

Deleted. I originally didn't look at the title of the forum here, and mistakenly assumed it was 401(k) deferrals, so my comment was inapplicable!

The only other item I'd add is whether 4% is "reasonable" and whether any portion of the 4% earnings after the balance is taken into account is still subject to FICA.