A single layer Blu-ray disc can hold 25,025,314,816 bytes. Assuming that you leave a 5% reserve for "overhead" like menus and such and encode the audio at 256 Kbps, that leaves you with an average video bitrate of 6,348 Mbps for your four hours of material. This is probably not going to look very good if the project is HD. If you really want to try to fit it all on there, then I would encode the video using h.264 instead of MPEG2.