C# store compare start position and actual position

Hello guys, I'm trying to make a script for a character to jump up to a certain height, then fall down. I'm using gravity to make it easier, but I'm having a little trouble finding how to limit the jump height.

I tried storing the starting y position of my player, then making a variable (toppos) that would be equal to 5 time the starting y as a limit. Then I want to compare the current y to that limit value and when they're equal the jump is off, and then when current y is equal to starting y again the jump can be used again. Still I must have done something wrong since no matter what I do and how high I go, the jump never goes off.

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent(typeof(Rigidbody))]
 public class Simple2DMoveController : MonoBehaviour {
 
     public KeyCode Left = KeyCode.LeftArrow;
      public KeyCode Right = KeyCode.RightArrow;
      public KeyCode Up = KeyCode.UpArrow;
      public KeyCode Down = KeyCode.DownArrow;
 
     private Vector3 startPos;
 
     private Vector3 toppos;
 
      public float speed;
      [HideInInspector]public float currentSpeed;
 
     void Start () {
 
     currentSpeed = speed;
     rigidbody.constraints = RigidbodyConstraints.FreezePositionZ|RigidbodyConstraints.FreezeRotation;
     }
 
     //to set starting position when touching a platform
     void OnCollisionEnter (Collision col){
         if(col.gameObject.tag == "ground") startPos = transform.position;
     }
 
     void FixedUpdate () {
 
         //bool to activate jump or no, declaration of toppos and check
         bool canjump = true;
         toppos = new Vector3 (startPos.x, startPos.y + 4, startPos.z);
         Debug.Log (toppos);
 
     rigidbody.velocity = Vector3.zero;
         if(Input.GetKey(Left)){ rigidbody.AddForce(-Vector3.right * speed, ForceMode.VelocityChange);
             }
         if(Input.GetKey(Right)){ rigidbody.AddForce(Vector3.right * speed, ForceMode.VelocityChange);
             } 
         //check if canjump is true
         if (canjump){
             if(Input.GetKey(Up)){ rigidbody.AddForce(Vector3.up * speed, ForceMode.VelocityChange);
                 //supposed to set canjump to false when toppos is reached
                 if (transform.position == toppos) canjump = false;}
             }
         //supposed to set canjump to true once ground is touched again
         if (transform.position == startPos) canjump = true;}
 }