Is Zakat obligatory on a sum of 4,000 US dollars—consisting of overtime pay and sales commissions—received at the end of the year, or is the Hawl (Zakat year) counted from the date of receipt?

For zakat to be obligatory on money, it is a condition that it reaches the nisab (minimum threshold) and that a hawl (full lunar year) passes over it. The hawl for receivables begins from the moment they alone reach the nisab, or when they are combined with other funds. If a full year passes, then all of it is subject to zakat. Receiving the dues late does not waive the zakat, because they are considered a debt owned by its rightful owner as soon as it becomes due.