I burnt 1 & 2 together. In order to do the quality had to be dropped to Long Play but it worked.
I've tried to burn 3 & 4 together but it won't let me. At first it shows the same error as with 1 & 2 (ie that the files are too big and the quality has to be reduced) but after i agree to reduce the quality it says it still can't fit them on a DVD! It shows the total amount of disk space used by 3 & 4 as hugely different to 1 & 2 even though 3 & 4 equal less than 1 & 2!!
If i follow then it's possible that a 55 minute clip could end up being bigger than a 45 minutes lip when burnt to DVD??? Once burnt to DVD it is "uncompressed" as it where? A file with more information will decompress bigger.