I can't think why I'd never seen this before. It must be well-known to travellers driving west on the A44 out of Moreton-in-Marsh towards Chipping Norton, positioned at a dog-leg crossroads about a mile and a half from the town. The Four Shires Stone once marked the spot where four counties met: Gloucestershire, Worcestershire, Oxfordshire and Warwickshire. Three still do, but the Worcs. boundary is now further down the road. (The nearby parish of Evenlode was once in an enclave of Worcs., cut off from the main county until 1931.) This was a meeting place for centuries, and there must have been previous stones now removed or sinking into the surrounding ditches. The current marker has a seventeenth century-ish look , but the local history society has it as 1909. (That looks about right judging by the lettering.) Such is the timeless appearance of beautifully weathered Cotswold stone.
A long time ago I'm sure my brother told me that there was once a similar marker in the meadows by the River Welland in Stamford, marking the meeting of Rutland, Lincolnshire, Northamptonshire and possibly the Soke of Peterborough, which I suppose will now be Cambridgeshire. He reckoned he once went swimming in the Welland, and deposited his clothes around the stone. Trousers in Rutland, shirt in Lincolnshire, vest in Northants, etc, etc. A likely story, but not impossible.