question

@Mark august, pairing a Multi with a BMV -particularly when using lithiums- is always a really, really good idea. The MultiPlus' built-in battery monitor is not great, and you want to make sure you take care of your new batteries. Definitely spend a little extra and get the BMV-712; this will allow you to set a number of parameters based on SOC and other conditions that you wouldn't be otherwise able to make with just the Multi alone.

Whether your 400Ah bank is sized correctly depends on a couple of things - if you're using Victron's drop-in LiFePO4 SuperPack, which is what it looks like you're using, then you're actually cutting it pretty close and should really get another 2 of them. The reason for this is that the Victron SuperPack 100Ah has a maximum continuous discharge rate of only 50A (.5C), so your 4 batteries together can only sustain a continuous discharge rate of 200A... and the MultiPlus 3000 can easily pull 300A or more (actually up to 516A of inrush current, but that's another story) if it's under load, so you could potentially see the Multi shut down with this configuration. Bottom line: if you're using the Victron SuperPacks and you anticipate needing to use the full 3kVA of the MultiPlus' capability, you should get another 2 of the same batteries to make sure you can sustain your Multi.

If you're not using the Victron SuperPacks, then whether or not you have a large enough bank depends on A: what brand of battery you're using and what its continuous discharge rate is, and B: what your anticipated loads are.

As far as kit, I mean this is a great start. Obviously you'll need fuses and breakers and wire and probably a few busbars and some other odds and ends. Don't buy the cheap Amazon knockoffs of anything... this is very important! I see a number of installations where all the big components are high-quality and then the connecting components are cheap... don't do that. Buy the name-brand Blue Sea/Bussman breakers and switches, and get a nice ANL 400A fuse between the batteries and the Multi on the DC side; Victron makes some nice ANL fuseholders if you want to keep with the brand, or Sterling also makes some pretty ones, or of course Blue Sea. On the AC side, you'll also want Blue Sea breaker panel(s).

As far as cabling, make sure you're getting good, properly stranded wire that's rated above your anticipated loads (general rule of thumb is anticipated load x1.25), that you account for your lengths (round trip) when sizing your wire, and that all your wire is fused appropriately; also make sure that you have good lugs and good connections everywhere, since a weak crimp or a loose connection will generate heat, which is the last thing you want. A handy wire size calculator can be found here: https://baymarinesupply.com/bosns_corner_wire_sizes

Overall you're off to a good start, other than your current battery bank being undersized for the Multi, though again that's only if you're using the Victron SuperPacks. 4 Battle Born 100Ah drop-ins would be fine, as they have a continuous output current rating of 1C (so total x4= 400A continuous output), I know ReLIon has drop-ins rated at 1C, a bunch of others as well. But if you've already bought the Victrons, just be advised that you'll want 2 more to be able to sustain your Multi if you have heavy loads on it.