I sometimes miss the most obvious things looking for whats not there. As a start I had several questions. Have you changed the fuel filters and bled the air out of the top of them? Is the fuel shut off, turned off? Does this tractor have a gravity flow fuel feed system? My 4500 does. There is a screen on the fuel intake above the cutoff valve, in the tank. If you disconnect the fuel line at the injector pump do you get good flow with the engine off? Is there a filter in the banjo fitting at the pump? Does the engine sound like it is starving for fuel before it dies? Does it sound like an air in system problem or a lack of fuel problem?
I changed the fuel filters on mine yesterday and even after bleeding air at the injector pump and the filter manifold I still needed to crack an injector line to get it to catch again. It is gravity feed to the injector pump so it can"t be a faulty fuel pump.
I hope there is something here that rings a bell. I hope what ever it is entails only a simple fix.