Q.What Is Wrong With My Acer Please. About 4ft High, Been In 4 Years. Fine 2 Days Ago But Now Leaves Shrivelled And Crumbly. Watere
Watered regularly. Next to a low wall

Certified GKH Gardening Expert
unfortunately, your photo did not come through.
If the tree gets rainfall at all, then it will not likely need extra water, depending on the type of Maple that it is. Acer is a very broad genus.
This leads me to believe overwatering to be the case.
Make sure to water, only, once the soil is completely dry down to a depth of about 3 or 4 inches.
I would treat with a fungicide, just in case.
These articles will help:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
Here is an article that will show you different species of Acer to compare which yours could be:
https://www.gardeningknowhow.com/ornamental/trees/maple/maple-tree-types.htm