PlayerMovement.cs

using UnityEngine;
using System.Collections;

public class PlayerMovement : MonoBehaviour {

    float speed = 4f;

    // Update is called once per frame
    void Update()
    {

        if (Input.GetKey(KeyCode.D))
        {
            transform.Translate(Vector2.right * speed * Time.deltaTime);
        }
        else if (Input.GetKey(KeyCode.A))
        {
            transform.Translate(-Vector2.right * speed * Time.deltaTime);
        }
        else if (Input.GetKey(KeyCode.W))
        {
            transform.Translate(Vector2.up * speed * Time.deltaTime);
        }
        else if (Input.GetKey(KeyCode.S))
        {
            transform.Translate(-Vector2.up * speed * Time.deltaTime);
        }
    }
}