I bought my first digital camera about six years ago for a trip to Banff, Yoho, and Kootenay. It was a 6.3 megapixel Canon 10D, with a cheaper Canon film body (so i could use the same lenses on both). Prior to that trip I had used film in 35mm, 6x7, 4x5" and some 8x10" for close to 50 years. In the mountains it had been my custom to use 35mm or 6x7 except when close to the road.
On that trip I took pretty much everything in both Kodachrome 64 and digital, and the 6 mp digital images greatly exceeded my expectations. Back home later on I made some side-by-side tests on tripod of 6MP digital shots compared with film on my Pentax 6x7 (scanned at a local professional lab). My impression of the results was that the 10D at 6.3 MP pretty much matched the resolution and image quality I got from the much bigger Pentax.
So my suggestion would be to take the D100 as a backup. It won't match 4x5 film, but it will do a fine job in situations where the 4x5 is just too much to carry. Then look into upgrading your digital when you can take time to study your options properly.
Good luck, and please post some photos when you get back. That is one of my favorite parts of the world.