Here in zone 5 we are close to the average last frost date. Which means there's still a good chance of frost for the next few weeks. You are in zone 4 so you probably have a couple weeks before the average last frost. Lots of websites out there for frost dates.

Also depends upon what you are planting.

My Walls of Water have been up since Saturday. I stuck my hand inside one last evening and it's nice and warm in there. I'm putting my tomatoes in the WOWs this week. I have lettuce and carrots in and radishes in. I'm waiting on anything else till our night time temps come up a bit. Everything is late this year!