Snapping object to .25 grid

I have this little tile drawing object for my little side project:

 public class DragTile : MonoBehaviour {
 //declaring some variables
     private bool mousedwn;
     
     
     void OnMouseDown() {
         if (!GameData.freemove){
             mousedwn = true;
         }
     }
     void OnMouseUp() {
         mousedwn = false;
     }
     // Update is called once per frame
     void Update () {
         if ((mousedwn) && (Input.GetMouseButton(0))){
             if (Input.GetButton("Control")){
                 this.transform.Translate(Input.GetAxis("Mouse X"),0,Input.GetAxis("Mouse Y"),Space.World);
             } else {
                 this.transform.Translate(Input.GetAxis("Mouse X"),0,Input.GetAxis("Mouse Y"),Space.World);
             }
         }
     }
 }
 

You may recognize it from my previous question. I'd like to be able to snapping tiles to grid when dragging when pressing down Control key, however I don't know how to do so as tile is pretty small (1/4 of world unit, scale 0.25 in x/z). Could you help me out?

I've tried to use following code in if (Input.GetButton("Control")):

 Vector3 pos = this.transform.position;
                 if (Input.GetAxisRaw("Mouse X")>0){
                     pos.x=pos.x+0.25f;
                 } else if (Input.GetAxisRaw("Mouse X")<0) {
                     pos.x=pos.x-0.25f;
                 }
                 if (Input.GetAxisRaw("Mouse Y")>0){
                     pos.x=pos.z+0.25f;
                 } else if (Input.GetAxisRaw("Mouse Y")<0) {
                     pos.x=pos.z-0.25f;
                 }
                 this.transform.position=pos;

but it didn't work (movement was giving strange and unpredictable results - you could move tile to the opposite side of map by moving mouse by 1 pixel).