What kinds of loads do you have that you need such large(re. expensive) cable? I have used #4 cable since I bought the boat in 1998. It came that way and I saw no reason to change. It has always started immediately, the new engine that is. I have nothing else that draws more than a few amps. I'm just asking, not being argumentative. I want it to be right.
Part of the problem are all the very short pieces, less than 12 inches. Can 1/0 bend? The only long run, about 15 feet, is from the golf cart batteries in the bilge to the bus(see pic). A real short piece in the bilge from battery to fuse. Then several short pieces on the panel in the picture.
I had the opportunity to be at the Landing School, one of the finest boat building schools in the country, on the day the systems instructor was having the students swap out different sized battery cables for starting engines. Hearing the difference in the way the engines started was simply amazing.
Good connections are a must, and can very often be a cause of voltage drop, but with #4 you really should have everything else at 150%. Before the ABYC existed as it does today boat builders did what they wanted with battery cable and "shot from the hip". Today those same builders, namely Catalina and Hunter use HUGE cable in comparison. Catalina today uses 1/0 battery cable on the M-25 Universal.
We also can't forget that there are many, many boaters who regularly start off the house bank and this bank. Others only do it in an emergency, to start the engine off the house bank but the fuse still needs to be sized to protect the wire and so it does not blow. #4 will start the motor but 1/0 or 1GA will start it much more efficiently. When the engines "jump to life" the starter see's less load for a shorter period of time.
House banks now need to be fused within 7" of the battery post under current safety standards. #4 wire is too small to fuse adequately and then connect to a situation where the motor might need to be started off this bank. The biggest fuse you could safely put on it for starting loads, with the wire in an engine space, is 135A and it would likely nuisance blow. Wire current carrying amapcity is de-rated in engine spaces and thus a fuse for #4 could very likely blow trying to start many small diesels. The same fuse on 1/0 could be 240A which is more than enough to start most any small diesel. You can go to 150% of this rating if absolutely needed but I generally try not to. Also many boats today have windlasses and other high draw devices like inverters so the large cable helps with this too.
Catalina finally saw the light on battery cable and our 2005 C-310 shipped from the factory with 1/0 battery wire on a Universal M-25 XPB. The 80's Catalina's shipped with #4 on the same basic engine. 1/0 wire is the spec for it on Catalina's electrical schematic for the C-310 W/ Universal M-25....
If starting new, and replacing the battery cables, I'd spend the money to go to 1/0 or at least 1GA. The difference between it and 4GA noticeable, your starter will be happy, and you'll be able to safely fuse the bank.
Universals requirement for the M-25 is:
4 Ft. Wire = 2GA
5 Ft. Wire = 1GA
7 Ft. Wire = 1/0
9 Ft. Wire = 2/0
11 Ft. Wire = 3/0
14 Ft. Wire = 4/0
Note that under no distance do they allow for #4. This is the wire length along the conductor not the "as the crow flies" distance.
I have seen engine compartment batteries with the switch right next to it that still had more than 6 feet of wire length to the switch and back to the starter & ground. You also can't forget the ground wire length as your starter circuit is a circular circuit from the battery to the starter and back to the battery.
Yanmars requirements are similar to Universal/Westerbeke in that the minimum wire size for short distances is 2GA, except they suggest a max voltage drop of less than 2% to the starter circuit...
If we take an older Yanmar 2QM with about 10 feet of battery cable and a 316A in-rush for the starter motor and 200A nominal starting load the math shows; 10 feet @ 2% or less VD would require 1/0 wire.
Sizing the wire bigger also allows for some degradation of connections or dirty terminals, which happens.