Does a faint line on 4 x home pregnancy tests which have not increased that much in darkness over 2 days indicate likely impending miscarriage due to low levels of HCG?

No - a faint line means there is not much HCG in your body. HCG levels double each day. A faint line means you are very early in your pregnancy. Retesting only one day later is not going to make much difference. If it is still faint in a weeks time, this means you are not increasing your HCG and your pregnancy will probably not continue. Hold off and try the test again in a couple of days.