Excellent, I'm glad to hear you successfully replaced two cartridge chips.
I am alarmed that Epson told you having a full channel not print since day one is normal- because it's ABSOLUTELY not normal, and means there's an issue with your printer- either in the head, damper, ink line, connector, or cartridge. If you have tried more than one cartridge, and have the same results, I would rule that out, and suspect it's an internal issue with the printer. We don't have a 4800 anymore, but I believe the #5 channel is LK- correct?
If nothing has printed from that channel since the printer was new, then I'm not confident that I can help you fix it via email (without seeing all the parts inside myself), and expect you'll need a tech to get it working- though, know they will automatically replace anything that could possibly cause the problem, including the dampers, print head, and possibly other parts, costing around $1,000 for them to get it working.
Yes, there are things you can do yourself, and may be able to fit it yourself (it's worth trying!)- I would start by replacing the dampers.
We have some printer maintenance info, which includes cleaning procedures as well as our recommended maintenance schedule, and when to replace parts, here: http://www.inkjetmall.com/tech/content.php?133-Printer-Cleaning-and-Preventative-Maintenance
You can get many parts from Compass Micro, and the repair manual from 2manuals.com
All the best~ Dana