You can't be reimbursed more than $5000 in calendar year (tax year) 2024.
Each employee's W-2 will show the dependent care benefit in box 10, it will be the actual benefit for the calendar year, regardless of whether the company uses a different plan year.
You will have to ask her employer how they will process her request. If she requests $4000, it will be up to the company whether they deduct $500 per month through June 2025, or if they deduct $333 per month (which would be the annualized amount, but would end up only deducting $3000 over the remainder of the plan year). If she asks for a full $500 per month, the company might refuse since that annualizes to more than $5000, or they might permit it, since she is enrolling for a partial year. It's up to them.
Form 2441 accounts for amounts left in the plan for the following tax year. For example, suppose she manages to have $500 per month deducted. Your combined FSA benefit (W-2 box 10) $4962, so you are still under the $5000 limit. If she is limited by her employer, it will be even lower, leaving you plenty of room under the $5000 cap.
In the event the total contributions/deductions were more than $5000, this will not cause a problem as long as you indicate the funds were carried forward to 2025 as permitted by the employer and not used for care in 2024.
For your wife's plan, you can only request reimbursement for expenses incurred after her plan starts.
