For a 4" concrete slab, how long of a run before an expansion joint is
necessary? 50', 100'?

At the expansion joints, what is the best way to maintain the two separate
slabs on the same level over time. Dowel and sleeve or keyway (tongue and
groove)?

For cutting of the control joints, I hear within 24 hours (green) and also
10 days ("cured") after the pour. So what is the best time to cut the
control joints and why is this timing so important?