Ahoy Bing and welcome.
First, check the possible causes that are cheap and easy:
~Are you 100% certain that #4 is at TDC when you're checking the clearance?
~Are you 100% certain that you're using the metric measurements given on your feeler gauge and that the blades aren't stuck together? Most feeler gauges in the US are given in inches, while our bikes' clearances are almost always given in millimeters.
Sorry if these sound like stupid questions, but I've made similar mistakes and have learned the hard way to check for the basic stuff before calling a machine shop for an appointment.
Next, these are the screw-type adjusters, right? If so, what happened when you attempted to loosen the adjuster screw more? If you cannot loosen it anymore, it sounds like these valve seats have reached the end of their service life. The only reason I can envision for the #4 exhaust valves having worn out more quickly than the others is additional heat. Possibly #4's carb was too lean for a period? How does the #4's sparkplug look compared to the others?
If this is the case, I've heard of some places that can add material to your valve seats and then re-cut them and the valves, but this would be almost as expensive as buying an all-new head; and certainly more expensive than searching every moto salvage yard in your state for a new head.
I hope it's one of the simple causes and I mean no offense when suggesting them. I can't believe that your valve seat would have worn so much after only 30,000 miles. I've got 32,000 miles on my '02 ZX9R and some of the valves still haven't required any adjustment at all.
Good luck and let us know how it goes!