There isn't as huge of a difference as the same interval in the 1990s.
In 1995 I had a 1980 CR80. HUGE difference: air cooled, disc brakes, fork travel/adjustability, twin shocks vs. single rear shock.
In 1997 I had a 1989 CR125. Not as big of a difference. Steel frame, conventional forks vs. inverted, ergos.
2018 CRF250R vs. 2004 - David Vuillemin commented recently to the effect that if it has a liquid cooled motor, single rear shock, similar ergos make older bikes similar to the newer ones.
The biggest thing when compared to your 2004 is the 2018 will have a fresh feel. Tight bearings, crisp motor, nice suspension. Also the 2018 is supposed to have a totally updated motor with more power plus the EFI tenability/ridability. Unless you start to get serious on the track however I think its a wash in every category except reliability.
As someone mentioned earlier - fast guys go fast, slow guys go slow.