It doesn't matter whether Shabbos starts at 4 o'clock or 8 o'clock -- you'll only be ready for Shabbos fifteen minutes before it starts.
Corollary to Wolf's Law of the Variance of The Start of Shabbos:
It doesn't matter whether it's 6:30 or 11:30 -- you'll be zonked fifteen minutes after the Friday night meal ends.
The Wolf