"平伸四肢" can be translated into English as "stretch out one's limbs flat" or more naturally as "lie flat with limbs outstretched".
Here are some example sentences to illustrate its usage:
After a long day of work, she liked to lie flat with her limbs outstretched on the sofa. （工作一整天后，她喜欢平躺在沙发上，四肢伸展开来。）
He stretched out his limbs flat on the grass, enjoying the warm sunshine. （他平躺在草地上，四肢伸展开来，享受着温暖的阳光。）