Okay - your question interested me as I recently did the search for a new scanner after my trusty old (read 'old') SCSI scanner died... It was a great, solid scanner (they don't make them like that anymore) but it was as slow as a waiting line at the Bank. In the end I got quite confused with all the new scanner models and decided that what I really needed was a photo printer instead - so I got the Epson 'all-in-one' RX510. It has a reasonable DPI scanner inbuilt and "WOW" - the scanner is fast, clean, and colour correct. I'm very impressed with Epson and more specifically their drivers for Mac OS X - which they update regularly. (This can be a problem with other manufacturers.) So, first things first, I'm glad you are only looking at Epson products, because seriously (for the Mac) they are the only ones to look at - (and 'no' I do not work for them...)
The two scanners you have picked are both pretty nice things - and regardless of either one you will be very happy with your purchase. I assume the 4990 model you are talking about is the PHOTO version and not the PRO version (which is top of the line and would be significantly more expensive than you have stated). But I must say, that is a very good price for the 4990 photo. On price alone, I would say go for that one.
Moving on to specifications - both units are almost identical. Like I said, you would be happy with either... But there are a few differences. The 4990 photo has slightly better optical density which (in theory) would give you better scans - although I doubt either of us would be able to tell the difference. Secondly, the 4990 photo supports Firewire IEEE 1394 as well as the standard USB 2.0 (the 4490 does not). These days that isn't really a big problem - but it is handy if you ever need to hook the scanner up to a older Mac that only has USB 1. Plus, Firewire can often be the 'consistently' faster interface bus. The third difference is speed. The 4990 photo is the faster performer at 12.3msec/line vs the 4490's 16.96msec/line. Long gone are the days when the flashing coffee icon suggested that you go for a coffee break while the scan was in process - which was a good idea considering that is how long it took (although I usually found myself stuck holding the scanner lid in place and staring menacing at that flashing coffee icon...). Both these scanners are fast. But if you want to recapture those lost seconds in the day - go for the 4990 photo. And besides it adds up - the 4.66msec/line difference in speed means the 4990 photo will scan much faster. The only other major difference in specifications is the 'reliability' and MCBF (mean cycles before failure) which is 100,000 cycles for the 4990 photo, and 36,000 cycles for the 4490. That's a big gap, and despite the two products being nearly identical this suggests that the 4990 photo uses more quality components and has better quality control in it's manufacture process. From a re-sale point of view the 4990 photo wins 3 times over.
The only feature the 4490 has that the 4990 photo doesn't is buttons on the front for scan, copy, email, and PDF, respectively. The 4990 photo only has one button (which I'm not even sure what it does... scan?) What this means is that the 4990 photo is much more software dependent than the 4490 - which brings up the next point. Although the included Epson scanning software that comes with both products is excellent and has many of the features only available to professional scanning software applications - the 4990 photo ALSO comes bundled with a copy of LaserSoft Imaging SilverFast SE 6. This is a professional industry recognised application - and even though it is only the entry level version (it could be upgraded) it does give you the choice. From my experience, a scanning device is only as good as the software and drivers that run it.
All in all, "Is the difference in price really worth it?" - absolutely!
If I was you, I would definitely go for the 4990 photo.
Anyway, you're a "graphic designer" (who probably more than anybody should know the dangers of judging a book by it's cover) but, lets face it... the 4990 photo LOOKS way better than the 4490, and plus it's got that "light on the top which indicates the progress" - priceless... I think I would have a hard time keeping a straight face and calling myself a graphic designer with the dodgy looking 4490 sitting next to my stylish Mac....
> If anyone has been impressed with my smart-arse review of these two products and would like to offer me a job - please let me know...