First off, point of confusion re: measurement;
A 4.7GB DVD (in binary measurement) will hold approximately 4.37GB (in decimal measurement). The difference is because you have both decimal and binary measurements. The capacity stated of 4.7GB is binary. When you look at the file size using Explorer, and most other utilities, it is reported in decimal. Remember that 1K is either 1,000 or 1024....
500K is either 500,000 or 512,000.... and so on.
It's risky to burn too close to the edge of the disc...the dye tends to thin out and the predisposition to burn errors becomes too great. This is true even on quality media like Taiyo Yuden and Verbatim. With cheap or poor quality media, it's that much worse. It's probably best to leave the default size at 4200 MB.
Hope this helps.