What is the most likely diagnosis for a horse with stiffness in all four limbs and petechial hemorrhages?

The most likely diagnosis for a horse with stiffness in all four limbs and petechial hemorrhages is purpura hemorrhagica. This condition is an immune-mediated vasculitis that can occur following infection with Streptococcus equi, the bacteria responsible for strangles. The characteristic clinical signs include swelling, and bleeding under the skin, leading to petechiae or larger areas of hemorrhage. The stiffness in the limbs can be due to the inflammatory response affecting muscle tissues or joint inflammation, which can also be secondary to the underlying immune response.

In contrast, while equine infectious anemia can present with similar symptoms such as weakness and anemia, it typically manifests with intermittent fever and more widespread systemic signs, not specifically stiffness or localized petechial hemorrhages in all limbs. Epizootic lymphangitis primarily affects the lymphatic system, leading to swelling and ulceration, but not typically with the systemic stiffness or petechial signs as described. Lastly, laminitis usually presents with severe lameness, but it would typically involve the hooves rather than presenting with systemic signs like petechial hemorrhaging. Thus, purpura hemorrhagica aligns most closely with the symptoms presented.