My van starts up then stops: would this be electrical, fuel filter, fuel pump, or what?

I have a 98’ Chevy Express Van…When I start it, it cranks over, fires up, and runs for about four seconds then dies as if it is out of gas. What could be the problem? I will give you some history that may (or may not) lead to the problem. BTW: it does have gas in it and I also added some “dry gas” to the fuel in case it was caused by moisture in the tank.

History: I was having a problem with the ignition switch. I almost replaced the switch, but I have anti-theft (the part is more expensive) so I was going to try to bypass the switch and install a button, but I haven’t gotten there yet.
I cleaned the connections to my relay box…THIS proved to be the, and or, one of the problems. With the switch disconnected from the steering column, I was able to start the van using the switch (and key in the “run” position), but as I mentioned, the van will run then quit.

Any suggestions?