The channel numbering the TV spits out is not accurate anymore and is just a data label / field in the transmitted signal. 4 isn't VHF anymore. 12 is actually high VHF on DTV 13. Etc.
If you're having trouble with 2 and 4 here locally, they're UHF now. 7 and 9 stayed VHF which for most people is actually more problematic. Only station in town that gained significant coverage area via engineering to do so is 31. 4 and 31 are the same power and same antenna gain from almost identical heights above average terrain. Both on UHF.
Ch 4 broadcasts from the TOP of that tower you're pointed at on Lookout Mtn and has a much better signal on UHF than 7 and 9 on VHF, who use a MASSIVE VHF antenna combiner system and broadcast from the front face of that tower in a cartioid pattern which lost them major coverage to the West to try to push VHF out onto the plains.
2 is somewhat north of the shared tower and is on a UHF channel now. 6 is downtown. 12 is on Squaw Mountain. 31 is north of Squaw between Denver and Boulder.
I suspect you have an old antenna meant for VHF only, when I see 2 and 4 are your problem channels. Or you have old tired coax that's awful at UHF (lossy) and a really long cable run.
I'm assuming an outside antenna here. Are you using rabbit ears?
You're getting enough signal from 31 shoved at you that it's getting through the antenna tuned for VHF or bad cable and if 7 and 9 are good, it's a good guess that you don't have a dual-band antenna or it isn't acting like one anymore.
If you can get 7 and 9, with new good quality coax and connectors and a good gain dual-band antenna, 4 should be a breeze. Seriously should be easy. Something is wrong there if it's not.
How's the signal strength bar look for 31? It should hammer in almost everywhere in the metro from the northwest.
See this page and the "post-transition" coverage maps as well as the "DTV channel number". Anything above 13 just like on the old TVs, is UHF.
http://transition.fcc.gov/dtv/markets/maps_current/Denver_CO.pdf
The maps were made in 2009 so ignore the transition transmitters. They're all gone now. Everyone had significant drawbacks to their coverage while they rebuilt.
If you want a tour of the Lookout site, I can probably arrange it. I have friends in low places up there. The backup transmitter for one of the stations up there is their " transition " transmitter and it's an oh-my-God enormous single tube transmitter.
Harris made all three stations up there sweetheart deals on their final transmitters and they're insanely cool modular power amps with individual 1000W trays that can be removed and replaced while the transmitters are on the air. Punch buttons on the touch screen, set module out of service, swap, punch button to go back to full power. Totally cool.
The combiner system for the VHF array looks like massive water pipe. They're enormous. They have the ability to interlock and remove one transmitter and bypass automatically in a failure mode and all sorts of nifty RF toys.
An amp *might* help. But it really can only help amplify what the antenna already can hear. The fact that you're pointed at a site with three transmitters, two VHF and one UHF blowing a megawatt into a nice omnidirectional antenna with lots of gain, tells me something is wrong with your setup for receiving UHF signals.
I forget where your house is but depending on surrounding terrain you could also be suffering from multipath (signal bouncing off of something else and two signals arriving at offset times at your antenna) on UHF. I don't think if you can boresight Lookout Mtn that this is the first thing I would look for though.
You could also have a local noise/interference source on UHF. I had a Linksys router go bad and spew hair all over a spectrum analyzer on VHF and UHF both once.
Anyway... Start by determining if you have a good outdoor dual-band antenna. A big Yagi or Log Periodic style will easily get you the Denver stuff and KCDO in Sterling off the back lobe and probably pull in the religious station on Lee Hill above Boulder. Out East here with the low noise floor they also pull in the stuff on Cheyenne Mtn in Colorado Springs pretty easy. I think the Palmer Divide will block you from that.
