Panels are all rated at the same Standard Test Conditions (STC). This makes it a little easier to compare panels but these are ideal conditions that are rarely met in real life. Consequently, it is somewhat unusual for panels to actually produce as much as they are rated for. However, 3200 W for 4860 (35% loss) does seem low.
There can be multiple factors:
Edit: Changed MPPT to PWM below
* Positioning is important. The panels will produce the most when the sun is hitting them straight on. If they are angled to the sun, production will be less.
* Even a little shading can pull down the output of the panels a lot.
* Heat can also significantly reduce production. If the panels are not mounted with plenty of ventilation around them, production can be impacted. particularly on hot days.
* A long run from the panels to the inverter can reduce the production if the cables are not over-sized to minimize voltage drop. (Your configuration is probably running ~40A at peak production. That will require 8AWG even for a short run.)
* What are the specs of the MUST PH18-5048 Plus charge controller? If it is rated for less than 4860W....it will throttle production back to what it can handle (assuming the voltage is not over what it can handle.... then it would just burn out)
* Is the MUST PH18-5048 Plus charge controller an MPPT or a PWM? Most All-in-ones have MPPT controllers but I have seen specs for some that have PWM controllers. The configuration of those panels probably is driving 120V at the controller input. That would be way too high for the sweet spot of an MPPT controller for a 48V battery (I am assuming 48V based on the model number)
* There can be significant output differences between good and bad solar controllers. Some just do a better job of harvesting the energy than others. I know nothing about the MUST PH18-5048 Plus converter but if it does not have a good MPPT controller, your production will suffer.
* Of course, if the battery is full and not taking current, the production will drop.... but that does not sound like what you are seeing.
One more thing: 405W is a big panel. Are they Bi-facial? If so they need to be mounted in a way that maximizes the light that strikes the back of the panel as well as the front.
Also note that all of the issues above are additive. If your panels are a little off angle, a little hot and have a little shading, the result could be a big reduction in production.