Sounds like the UHF ferquency that your
Sky box is using is too close to that of Channel 4.
Don't have a
Sky box but you are looking to change the UHF output frequency which must be on a menu somewhere. Someone here will know.
You will have to watch that when you change it you don't move it closer to another broadcaster ferquency of if you have a video that frequency.
This may help...
UHF Frequency is the output frequency that each peice of equipment will transmitt on, The UHF frequencie band starts at ch 21 and stops with most at ch 69 so, if for instance you recieve your terrestrial tv pictures from winter hill (granada), the ch that BBC1 taransmitts on is ch 55, therefore you cannot have your video or satellite transmitting on ch 55 because you will get interferance between the two things both trying to use the same frequency, ( this is called cross modulation ) also you cannot use channels either side of a terrestrial transmision e.g. ch 54, or 56 for the same reason, most new equipment will be fully variable from 21 to 69 so you may need to find a clear ch to put your sky on,
Sky's modulator output (UHF) frequency can be changed by pressing : Services / 4 / 01 select / 4..
On this menu you can alter the RF output anything from ch21 to ch69 and also turn on the second RF output voltage to power the triax 4 way splitter ( remember that if you make any changes you will need to go to save new settings and press select)
Your Scart is a direct connection and doesn't use this so that's why it works OK through it.