404 Jeffery throws a 400grn bullet at 2125 in its original cordite loading. The bolt action version of the 450/400 cartridge that was generally reckoned to be about as close to an "all round rifle" as possible, if such a thing exists.
An alternative to costly double rifles handling the 450/400. It's a superb Dangerous game cartridge.
Heavy bullet, moderate velocity, something I've come to love. Forget all your hyper velocity, light bullet stuff, these cartridges with heavy bullets and moderate velocities have been getting the job done for almost a century and in some cases more ( e.g. the .303 ). Oldsarge has got it spot on.

I dabbled in the "Every last foot per second" arena and learned that very few loads were as effective as a good heavy bullet at moderate velocity. My first red hind was shot with a Charger Loading Lee-Enfield ( Long LEE ), using a 174 grn Hornady round nose soft point at approx 2300fps. She never knew what hit her, instant kill and no meat damage.

My first sporting rifle was a Parker Hale modified No4. Sporting stock and cut down barrel with a modified 5 shot mag. They sold new for around Ł50. Beautiful rifle and did tremendous work for me using the Kynoch 174grn load ( I think the components were made by Norma ). Regret ever selling that. I own a No4 Mk1 as well as a Mk3 SMLE and an M1910 Ross, wish I could get another Parker Hale to complete the collection. Go ahead with your Mk2 No4 and enjoy, you'll never regret the purchase. If you can find a good .303 sporter at the right price, grab it, you'll never look back.

By the way you are right you don't need to be old to enjoy the .303, I started at 13! I'm 46 now and don't consider that old, I'll leave you guys to debate the fart bit!!!