How can I get 4 joints IK from 6DOF arm? [closed]

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Hi everyone, Now I am doing 6dof arm path planning. I need to control the robot arm from one position to another position. I first have to get the IK and then plan a path. The getik programs are as follow. I just hope to get IK from 4 joints(other 2 joints are set to zero). I am wondering what should I do? And another question: what is the principle of kinematics_msgs::GetKinematicSolverInfo?

kinematics_msgs::GetKinematicSolverInfo::Request l_request,r_request;
kinematics_msgs::GetKinematicSolverInfo::Response l_response,r_response;
  kinematics_msgs::GetPositionIK::Request  l_gpik_req;
  kinematics_msgs::GetPositionIK::Response l_gpik_res;
    quat = tf::createQuaternionFromRPY(phi*180/PI,theta*180/PI,psy*180/PI); l_gpik_req.ik_request.pose_stamped.pose.position.x = x; 
 l_gpik_req.ik_request.pose_stamped.pose.position.y = y; 
 l_gpik_req.ik_request.pose_stamped.pose.position.z = z; 
 l_gpik_req.ik_request.pose_stamped.pose.orientation.w =quat[3]; 
l_gpik_req.ik_request.pose_stamped.pose.orientation.x =quat[0];  l_gpik_req.ik_request.pose_stamped.pose.orientation.y=quat[1];  
l_gpik_req.ik_request.pose_stamped.pose.orientation.z =quat[2];
  if(l_ik_client.call(l_gpik_req, l_gpik_res)){
    if(l_gpik_res.error_code.val == l_gpik_res.error_code.SUCCESS)
            ROS_INFO("Solution for Left Arm Found );
      else
            ROS_DEBUG("Left Arm Inverse kinematics failed");
        }

thanks advance