You mention struts in your thread title and springs in your post. Which one? Make up your mind or we will ignore you. Just kidding...
First off, do you want to use your stock '98 struts?
-If so, you would need '04 WRX strut mounts or top hats to make the '04 WRX springs to work.
'04 WRX struts only.
-Don't do it because the shorter WRX struts are not design to work with your stock springs.
At all best, try to use WRX wagon springs because of the higher springs rate in the rear. You can use a complete WRX takeoff from either year range 02-03 or 04-05. The rear springs and strut mounts are different between those two sets of productions so avoid interchanging them. I will also caution you on the brake line brackets.