Is it possible to do an fuel injector flush on my 4runner first then change the spark plugs at a later time?

yes that's fine I can save you the cost of any fuel injection cleaner. Use chevron super unleaded with techron or use the regular. I have used chevron with techron for many years and have never been to the mechanic or anywhere else to put a bunch of crap like fuel injection cleaner in my car. Chevron costs a little more because they use cleaner burning chemicals to boost octane plus the add techron. This is a fuel injector cleaner you can buy in the stores as well.

Those cheapo gas stations use cheaper more contaminated chemicals that often have water in them and will go through gas filters and leave hardened deposits through out the fuel injection system. One tank of chevron will help slowly start to dissolve these deposits and if you use it regularly it will keep your injection system clean and you will no longer need to add harsh cleaners to your engine. Its better to keep the engine clean than go back and try to clean it out later. Your going to pay one way or the other I guess so it all depends on what you want to do.

In my opinion fuel injector cleaner is a waste of money. If you use good quality gasoline then the gasoline already has detergents in it already. My car has 332,000 miles on it and has never had fuel injector cleaner in it.

that's a better idea, as the when you flush the system , their will be more carbon and gunge going throe the engine , changing the plugs after will actually be better