Get a timer, inexpensive one will work very well. They are approx. $15 (and up from there) here in Canada, and probably less in US. I have quite a few of them and wouldn't be without...I keep many plants in dark basement over winter, and couldn't possibly make sure lights would go on and off regularly without timers.

If you don't have time to find a timer go dark for 4 days. It's not many days and better to play it safe. Seedlings easily burn under too much light.

Skip the water too. Watering when dark is bad news because the evaporation and plant water usage will be way down.

Yep, go dark. Often, I move as many pots as possible near the windows, so they can get as much light as possible. If you can't do that, no issue. I've left plants in the dark (the room had windows, but shades closed) for 7+ days.

Though it is much easier to get a timer!!

Yes, unfortunately, there just won't be time to go acquire the timer. We were already cutting it closer than we should have been, and now we have the extra time of getting the dogs to the "sitters" house... So, dark it is. They also won't have any water as there won't be anyone coming over to water them... (the neighbor is watching the outside of the house - not the inside :) )

Lesson learned, when we get back I'm buying a timer.

Have fun on your trip!

When you get back, I recommend finding a mechanical timer with two three prong outlets for best flexibility and safety. Skip the electronic ones because they are usually terrible.

As Rina says, they are cheap and often I've gotten them on sale.