Updating from cs2 to cs3
With 4K video, 30 fps Raw bursts and additional controls, it's a big step forward from its predecessor. The 90D is essentially the DSLR version of the EOS M6 Mark II mirrorless camera that was introduced alongside it.
In your use, you don't need Lightroom--Bridge is sufficient. Neither enhancement alone would be sufficient reason for upgrade, but the difference between Bridge CS2 and Bridge CS4 is (IMO).
Why not download a trial version of CS4 and try it out?
That's really the only way to know how the new features work and whether it's worth the cost to upgrade. Although I think if I had started using Lightroom before upgrading I might have just gone to the new version of Elements. This allows filters to be applied and objects scaled without affecting the original image resolution or detail. I don't think there was that much in CS4, bridge works a bit better and has improved filtering.
From what I've read CS4 is pretty bad about eating machine power.
hth There are really to many enhancements in CS3 and CS4 to comment on ... The bottom line is that there have been significant improvements to all aspects of the product (ACR, Bridge, Photoshop) since CS2. has some great classes to get you up to speed on all of the new features.