ב"ה, י"ט תמוז, תש"כ
ברוקלין.
שלום וברכה!
... במש"כ ע"ד אמירת ד' מזמורים הראשונים דתהלים כו' - כ"כ בקצש"ע סקנ"א (מס' יסוד יוסף) קצה המטה שעל מטה אפרים סו"ס תקפג ועוד, ולדידן (ובפרט שאין אומרים תהלים קודם חצות לילה - מלבד בר"ה ויוהכ"פ) עצ"ע ואכ"מ.
B"H, 19th of Tammuz, 5720.
Brooklyn.
Shalom u'Vrachah!
Regarding what you wrote about reciting the first four chapters of Tehillim at night—so it is written in Kitzur Shulchan Aruch section 151 (from Sefer Yesod Yosef), at the end of Mateh Ephraim section 583 and elsewhere. However, for us (especially since we do not say Tehillim before midnight—except on Rosh Hashanah and Yom Kippur), this matter requires further clarification and this is not the place to elaborate.
Summary
The Rebbe notes that reciting the first four chapters of Tehillim at night is discussed in halachic sources but points out that Chabad custom generally avoids saying Tehillim before midnight except on Rosh Hashanah and Yom Kippur; further clarification is needed.