  We consider dynamically convex (star-shaped) domains in a symplectic vector
space of dimension 4. For such a domain, a "Hopf orbit" is a closed
characteristic in the boundary which is unknotted and has self-linking number
-1. We show that the minimum action among Hopf orbits exists (without any
nondegeneracy hypothesis) and defines a symplectic capacity for dynamically
convex domains in four dimensions. We further show that this capacity agrees
with the first ECH capacity for dynamically convex domains. Combined with a
result of Edtmair, this implies that for dynamically convex domains in four
dimensions, the first ECH capacity agrees with the cylinder capacity.
