i want to go from sheung shui to the hong kong airport early morning at 4:00 am what is the best way to go

Of course taxi should be the best way! Except that, overnight bus is recommended!

If you wanna leave Sheung Shui at 04:00, overnight bus no. N42A happens to operate from Sheung Shui Railway Station to HKIA at 04:00!

If you need to arrive at HKIA by 04:00, I suggest you take overnight bus no. N73 at Sheung Shui to SHA TIN CENTRAL BUS TERMINUS, where transfer to bus no. N42 to HKIA directly then!