i have had my chain which is a shimano HG73 9-speed for 4 years with 1,000 miles PER year. I dont see or feel anything wrong with it except that it sounds a little noisy no matter how clean or lubed it is.

i see posters here saying 3,000 miles is enough for a chains life. should i change already?

This topic has been covered many, many times. A Shimano chain can be measured with a precision scale for elongation. If the center to center distance between 24 pins reaches 12-1/16", it has elongated by .5% and should be changed. Unfortunately, that alone does not guarantee that a new chain will not skip on one or more of the most used cogs. With 4,000 miles on the chain, it may already be too late and you may need a new cassette to go with that new chain.

In which case, if the used chain is not currently slipping, you might as well keep using it.

It probably is too late to just change the chain without having major skipping on the smaller most used cogs. I would measure the chain elongation as DaveSSS recommends. If it's 12-1/6" or so and there are no skipping problems, just keep using it. If it measures 12-1/8 or more, change it and the cassette to keep from wearing the chainrings excessively.

I have a Shimano 9-speed chain on one bike with 8000 miles and it measures exactly 12-1/16" over 24 pins. I'm going to change it reasonably soon but it's still shifting and running fine.

Grab a link that is currently resting on the chain ring (preferably, the link furthest from the rear wheel). Pull on it. If more than three links lift away from the chain ring your chain needs to be replaced (this doesn't mean they clear the teeth entirely, if they do, well, you've got bigger problems than just your chain - just that you can pull the links from the surface of the chain ring).