I am assuming you are starting them in pots, and then planting them in the ground when it warms up? I'll tell you what I do, but I don't claim to be an expert.

1) When to start them indoors in Zone 4? Where to start them? (basement, porch, etc)

I am in zone 4 WI, and usually don't start mine until mid April. (though I am starting some earlier this year) I usually put them in the ground the first week of June.

Dahlias like it warm. So to wake up the tubers and get them growing I put them in trays of moist potting mix and place them on top of my fluorescent shop lights, (lights that I use for growing seedlings) Any warmer (70-75 degrees) spot will work. I do this to ensure the tuber is going to grow. I don't want to waste time and space if the tuber is a dud.

2) Planting medium and depth?

After they have spouted - I pot them up. Any decent potting medium will work. I use a variation of "Al's 5-1-1" (see the container forum). I use 4" square biodegradeable pots to start them, so I can only plant them 3" deep (When they get planted outside the whole pots goes in 6" deep). I use smallish pots because I start close to 100 and they take a lot of space. A few really huge tubers get started in larger pots.

3) Do I water them?
I moisten the potting mix well before it goes into the pots and then don't add any additional water until the sprout is above the soil. Occasionally there are really slow growing ones that need a little watering before then - but keep it light.

4) When do I move them to a location that receives more light?

As soon as they poke up through the soil they need as much light as you can give them or they will be tall spindly weak plants. I put mine under fluorescent lights. Your 3 season room may be fine if it gets enough direct sun.

5) Which set of leaves do I pinch back when they begin growing?

Top the plants after they have at least 3 sets of true leaves. Because I plant them deeper when they go outside, I measure 6" from the bottom before I start counting my sets of leaves. (often the first couple of sets are going to be buried)

Have fun !

Thank you! I seem to remember starting them in pots, and keeping them there until I planted them out. Perhaps adding soil once they sprouted (definitely putting them on the porch at that time).

Mandolls, when you say mid-April is that when you start them sprouting or do you sprout them earlier?

I'm in a Mpls suburb. I plant my tomatoes and dahlias around Memorial Day.

This year I am sprouting them earlier - I got them all to eye up already and have them in pots, only two have poked up though the soil yet (planted about 3"deep)

Last year I don't think I even unwrapped them until mid April, so I am about 3 weeks earlier this year - I am hoping it won't be to early. The bigger they get the trickier it is to keep them happy and healthy inside. Last year, starting mid April, my plants were between 6" & 18" when I put them in the ground.

I read on another site to start tubers indoors using a soilless potting mix? It sounds like you have used soil with no issues? I'm anxious to get them going, but I don't want to mess this up. Overthinking this as usual : )