The 4th Mounted Brigade previously known as the 2/1st South Wales Brigade was a second line yeomanry brigade of the British Army during the First World War. Raised after the declaration of war, it was a mirror formation of the first line South Wales Mounted Brigade. It had under command the 2/1st Pembroke Yeomanry, the 2/1st Montgomeryshire Yeomanry, the 2/1st Welsh Horse and the 2/1st Glamorgan Yeomanry. All of which were converted in cyclist units in 1916, the brigade, never having seen any active service, ceased to exist.