Will a 60-70A fuse at the battery have any effect (loss) of voltage/amp in the wire, vs. using a 100A, other than the obvious of a more protected wire?
Would 4 or 6 power wires ran together, but openly (no wire loom usage, nor twisted) and supported by wire cushion clamps and wire ties constitute a bundled description?
Now that you have the short answers, here is a bit of discussion.
Q1/A1 Elaboration: The ampacity for AWG 4 in machinery spaces in a bundle of up to three current
carrying conductors is 95 amps. The National Electrical
Code (NEC) is used for the basis of the ABYC Standards. Both of these were utilized by Blue Sea Systems in developing their fuse selector. We are permitted, for a variety of reasons, to use a larger over current protection device (OCPD; fuse or circuit breaker) if the ampacity of the conductor is less than a commonly available OCPD. That said, I would install either:
> an MRBF (75A p/n 5180)
> an ANL fuse (80A p/n5124)
> or a MIDI fuse (70A p/n 5254)
with their associated fuse holder as the first OCPD off the battery or bus bar. All of these have an AIC >/= 5000A and are appropriate for this service
Q2/A2 Elaboration: The NEC and the ABYC provide tables for derating conductor ampacity based on the temperature of the environment
and bundling. The ampacity rating of the cable is limited by the capability of the insulation
to remain solid and insulate the conductor. Conductors marked as "Boat Cable-BC5W2-UL 1426" have a temperature rating for a single
conductor in free space of 105C. Bundling of conductors inhibits the ability of the conductors to release the heat naturally generated when a current is passing through them. The NEC utilizes derating factors based on environmental temperature and how many current carrying conductors are bundled. For DC circuits, the ABYC has only two tables for derating based on temperature and number of conductors bundled while the NEC's derating factors have a much finer granularity. The ABYC is currently (no pun intended) working on improving on the derating factors published in the E-11 Standard.
Hope this long winded explanation helps.