Resizing to very small

Hi all, I have a PSD that I have built with 4800 x 4000 pixels; 300dpi and my customer wants to have it in logo size, so quite small for a letter head. I do not quite manage to maintain the quality of my bigger file. Would anyone have the expertise in PSD of how to resize to such a smaller size (approx. 2,5 inch to 1,5 inch) maintaining the good quality? thx in advance, Kristina