The wedding is at 4 PM so technically wouldn't the PC be too much? I fear that I will be way overdressed.
Regardless of the time, unless specified otherwise a PC would be way too much; as inappropriate and out-of-place as wearing a full wetsuit with tanks, mask, & fins.
What does the invitation say? Very many formal weddings around here start before six; however nobody outside of the wedding party is expected to dress black tie. After six, not all weddings are formal. A less formal jacket & tie like the Argyll would be fine (unless of course it's an informal garden or beach wedding, in which case loose the coat). It's their wedding; dress according to your host's expectations; when in doubt, ask. That's why you're called a "guest".