class_docstr: Stack previous four frames (must be applied to Gym env, not our envs).
class_docstr_tok: ["Stack","previous","four","frames","(","must","be","applied","to","Gym","env",",","not","our","envs",")","."]
code: def reset(self) -> np.ndarray:
      observation = self._env.reset()
      for _ in range(FrameStack4.STACK_SIZE):
          self._frames.append(observation)
      return self._generate_observation()
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import collections
  from typing import Any, Text
  import gym
  import numpy
  from tf_agents.environments import py_environment
  from tf_agents.environments import wrappers
  from tf_agents.trajectories import time_step
label_desc: Use image node_id 4 for calling the FrameStack4 obj's underlying member method code with example usage: obj.reset() and returns: self