The map reference is Y4.

地图编号为Y4。

《牛津词典》

To combine the scheme and classification maps for a deliverable, a higher-level map can refer to both maps using a DITA map reference (see Figure 6).

要把方案图和分类图组合起来形成可以发布的内容，可以用dita图引用，在更高层次的图中引用这两个图(请参阅图6)。

youdao

The route is best seen by reference to the map.

参照地图能将这条路线看得十分清楚。

《新英汉大辞典》

The signs contain the place name and a map reference which people can give coastguards when reporting an incident.

BBC: South West Coast Path gets 500th 'life-saving' sign

Each missile comes with a terrain contour matching radar, which compares a stored map reference with the actual terrain to fix the missile's position.

BBC: Tomahawk cruise missile