"平伸四肢" can be translated as "stretch out one's limbs flat" or more naturally as "lie flat with arms and legs outstretched".
The first option is a more literal translation, while the second one conveys the meaning more vividly and naturally in English. Depending on the context, you could choose either one. For example:
She lay flat with her arms and legs outstretched on the grass.（她平伸四肢躺在草地上。）
He stretched out his limbs flat on the bed after a long day.（忙碌了一天后，他平伸四肢躺在床上。）