How can I subscribers multiple topics to get data at the same time (python)?

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Hi, Am new to ROS and I need a little help. There are 4 topics (robot0/sonar1, robot0/sonar2, ...) of same type (sensor_msgs/Range). I need to process these 4 values together.

#!/usr/bin/env python
import rospy
from sensor_msgs.msg import Range



def callback0(data):
    rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.range)
    sonar0 = int(data.range)
def callback1(data):
    rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.range)
    sonar1 = data.range
def callback2(data):
    rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.range)
    sonar2 = data.range
def callback3(data):
    rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.range)
    sonar3 = data.range
def listener():
    # In ROS, nodes are uniquely named. If two nodes with the same
    # node are launched, the previous one is kicked off. The
    # anonymous=True flag means that rospy will choose a unique
    # name for our 'listener' node so that multiple listeners can
    # run simultaneously.
    rospy.init_node('listener', anonymous=True)
    rospy.Subscriber("robot0/sonar_0", Range, callback0)
    rospy.Subscriber("robot0/sonar_1", Range, callback1)
    rospy.Subscriber("robot0/sonar_2", Range, callback2)
    rospy.Subscriber("robot0/sonar_3", Range, callback3)
    rospy.loginfo(rospy.get_caller_id() + "I heard %s %s %s %s", sonar0,sonar1,sonar2,sonar3)
    # spin() simply keeps python from exiting until this node is stopped
    rospy.spin()

if __name__ == '__main__':
    sonar0=0
    sonar1=0
    sonar2=0
    sonar3=0
    listener()

The line

rospy.loginfo(rospy.get_caller_id() + "I heard %s %s %s %s", sonar0,sonar1,sonar2,sonar3)

is not even executing. But I was able to see the result of individual callback. What is the best way to do this? I need an another function that has to process these 4 values.

Update:

Just like you said I am only getting the

rospy.loginfo(rospy.get_caller_id() + "I heard %s %s %s %s", sonar0,sonar1,sonar2,sonar3)

line executed once. What is the best way to do this? I want to store all the values in an array and process once all the four values are received. Then repeat this process again.