I'm no expert by any means, but there are two things you need to understand when you start. You choose the diameter of wire you use by the maximum current it will need to carry as well as the voltage drop that will occur over the length of the wire. You then choose the value of the fuse for that wire based on the amount of current it can safely handle. In short, the device on the other end of a wire and the distance to that device tell you how big your wire needs to be. Then, the size of the wire tells how what fuse to use.
You may already know this, but it's worth mentioning that in the American Wire Gauge (AWG) system of labeling, as the diameter of the wire gets bigger, the number gets smaller. In real life, I've only seen even numbered gauges. Common ones used in solar go from 10 AWG, 8 AWG, etc. down to 0 AWG. Then you start adding more zeros with 00 AWG (commonly written as 2/0 and pronounced two-aught), 0000 AWG (also 4/0 and four-aught), etc.
Let's step through it with some example numbers. For the wiring from your bus bars to your inverter, divide 2,000 watts by the low voltage cutoff point of your inverter or BMS, whichever is higher. For the sake of easy math, I'll use 10 volts in this example. 2,000 watts divided by 10 volt = 200 amps, so you'd need to use a wire that can handle that current like 2/0 (size 00 AWG). The 4 AWG wire you mention can carry 135 amps if it has 90 degree C insulation or 180 amps if it has 200 degree C insulation. I don't think a 250 amp fuse on a 4 AWG wire is providing any protection. The wire will heat up and catch fire before the fuse ever blows. This is before taking into account voltage drop due to the resistance of the wire, so be sure to use a voltage drop calculator with your chosen wire diameter and length to see how much voltage you'll be losing. You may need to go up a step or two if you're running more than a couple feet. You'll also need to use the current required by your DC-DC charger (50 amps) and the wire length to choose the size for the wiring between your busbars and the DC-DC charger. The wiring between the starting battery and the DC-DC charger needs to be sized for safety as well, but you'll also want to use the largest wire you can here to avoid too much voltage drop.
You mentioned your battery bank. I assume you mean you'll have a 400 amp/hour battery bank. Amps are a measure of current, but amp/hours are a measure of capacity. Four 100 amp/hour Battleborn batteries in parallel, for example, would be a 400 amp/hour battery bank, but it could output 800 amps of current for 30 seconds or 2,000 amps for 1/2 a second.
Here's an example system that's kind of similar to what you're talking about.
https://www.explorist.life/2000w-520w-60a-budget-friendly-camper-wiring-diagram/
The DC-DC charger is only 30 amps, so you'd need a larger wire fire that. It's intended for a van. If your wires are going to be longer, you'll probably need to go a little thicker. Check a voltage drop calculator for that.
