Trading ETF’s in the After-Market?

Does anyone have experience with trading ETF’s in the after-market? I’m a newbie at this.

Toward the end of today’s normal session, I went long an ETF at a price that ended up being its closing price, 18.21. But when I pulled a quote at Yahoo Finance just now, they reported the higher close of 18:39, with a time of 4:18 PM.

When I pull info on trading hours for ETF’s at the various exchanges, IB lists the following schedules:

AMEX, 09:30 - 16:00
CBSX, 09:30 - 16:15
CBOE, 09:30 – 16:00
NASDAQ, 09:30 – 16:00 (Normal), 16:00 – 17:30 (Extended)
NYSE, 09:30 – 16:00

Is ETF trading one of those situations where “It ain’t over, until it’s over”, and it ain’t over for ETF’s until 17:30? That’s an hour and a half later than “normal hours”.

Meanwhile, do I assume that any stop written as a day order would have expired at 4 PM? Is this what happens?

Thanks in advance, Charlie