"You are correct with the .380-200, but the Mark IV was adopted in 1899 according to what I read on the subject. Is this wrong??"
OK, you're talking a completely different Mk IV Webley revolver.
The one your Grandfather purchased was a Webley Mk IV in .455. This is a very different gun from the Enfield/Webley Mk IV in .380-200.
The Mk IV in .455 is sometimes called the Boer War Model because it was introduced around the start of the war and was used quite extensively in the conflict.
Even though it was significantly beefed up over previous Wekbley marks, the Mk IV still wasn't quite strong enough to stand up to a lot of shooting with smokeless ammunition, which led to the adoption of the Mk V in 1913 (the last of the Webley "birdshead" grip revolvers) and the Mk VI in 1916.
Word of warning...
NEVER EVER shoot your Webley with .45 ACP ammo, even if it has been converted to take that round by altering the cylinder. .45 ACP ammo generates significantly higher pressures, and while the Webleys look more than strong enough to take it, they're simply not.