This will happen if channel 4 is not tuned in.
Try running autotune to re-tune all channels again.
If that does not work, try manually tuning in the channel, or try using an antenna booster if the signal is weak.
If manual tuning does not work then autotuning won't work either.
You may not be receiving a strong enough signal for channel 4 - each channel signal will have a different strength, so you may receive some channels but not others.
You will need to try fitting an antenna booster:
Yes, this is a common problem .The strength of the signal can vary due to environmental factors, changes in the position of your antenna, etc.
Try the booster and let me know if it fixes the problem for you :)
I would really like to help resolve this problem for you.
The fact that you changed to channel 39 is co-incidental.
The problem that you are experiencing, where a channel that was working previously suddenly disappears, is a common one and is usually caused by the signal strength weakening. You can fix this by boosting the signal again - to do this, simply fit an antenna booster - you can see what an antenna booster is by clicking on this link:
A high rating for my service would be greatly appreciated :)
This was the first point I mentioned, where I asked you to re-tune all channels again, but you said it did not work!