It reads that is does not need to exceed four digits, not that you must truncate the account number at all, if you do not chose to do so.
229.13(g) Notice of exception--
(1) In general. Subject to paragraphs (g)(2) and (g)(3) of this section, when a depositary bank extends the time when funds will be available for withdrawal based on the application of an exception contained in paragraphs (b) through (f) of this section, it must provide the depositor with a written notice.
(i) The notice shall include the following information--
(A) A number or code, which need not exceed four digits, that identifies the customer’s account.