Accessing data from 4 seconds back

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Hi all, I am trying to create my own acceleration node. The idea is to have the difference between 2 velocities, every 4 seconds for example. I know how to get the value of the velocity at the current time. but how do I get the velocity 4 seconds back? the value should be accessible as the program runs, or as bag files are being played.

Below is my code:

#!/usr/bin/env python
import roslib; roslib.load_manifest('kee_use_cases')
import rospy
import math
#from std_msgs.msg import String
from std_msgs.msg import *
from nav_msgs.msg import Odometry
from geometry_msgs.msg import *


    def callback(msg):
        rospy.loginfo("Received actual forward velocity <</odom>> message! The value is %f" %msg.twist.twist.linear.x)
        if rospy.Time.now() == 0:
            vel_init = msg.twist.twist.linear.x
            buffer_function(vel_init) #?
        else:
            #how do i get the the vel_init?
            vel_last = msg.twist.twist.linear.x
            acceleration = (vel_last - vel_init)/4
            talker(acceleration)

    def buffer_function(vel_init):
        return vel_init


    def talker(acc):
        rospy.loginfo("x_acceleration: %f"%acc)
        pub = rospy.Publisher('x_acceleration', Float64 )
        pub.publish(acc)

    def listener():
        rospy.Subscriber("odom", Odometry, callback)

    if __name__ == '__main__':
        rospy.init_node('x_acceleration', anonymous=True)

        duration = 4
        while not rospy.is_shutdown():
            try:
                listener()
                rospy.sleep(duration)
            except rospy.ROSInterruptException:
                pass