[Antennas] why is this?

[DJED1 at aol.com]

No surprise.   Remember, the antenna has widely different impedances 
depending on the band, which is probably not close to a match for the 450 ohm line.   
So the fact that the 4:1 balun would match better to a 450 ohm line doesn't 
matter, because the impedance at the tuner is whatever the antenna is 
transformed through 75 feet of 450 ohm line, and is probably not close to 450 ohms.   It 
would appear that the impedance at the tuner was within the range of the 
tuner with the 1:1, and not so with the 4:1.   You've got several choices:   go 
back to the original balun; change the length of the feed line and hope things 
change for the better; get a tuner with a wider range.   I understand that some 
of the internal tuners have a limited range, but I have no personal experiece 
to back that statement up.   I'd go with the first solution.   Good luck
Ed WB2LHI