400w/12v=33.33 amps.

That is your theoretical max amperage so yes a 20 amp charger is undersized for that array. If the 20 amp charger is designed to handle being connected to more solar wattage than the can be passed along to the batteries (some are, some aren't) than you are fine providing that you are ok with curbing some of your solar charging production in good sun/low battery conditions.

A 40 amp charger will result in more charging in some common conditions.

thank you

Solar charger will shutdown once the battery voltage reaches their normal voltage of 13.6 volts.
Each solar panel can put out a maximum of 6 amps. Each panel has a two rating . One is open circuit DC which normally is about 20 volts. The other is max short circuit current which normally run at about 6 Amps. Since under normal conditions solar panels don't run at max values . The 20 Amp should be OK . You can use the 40 Amp if you plan to add or upgrade the solar panels later.
My own experience with stationary solar system is that on a bright sunny day ,and with solar panels at the correct angle . Our output was 5.5 A short circuit current with an open circuit voltage of 18 VDC. The panels where rated for 120 watts . Our actual output was a 100 watts .

George, Thank you for your input. I will keep the 20A for now, will upgrade later.

With a mppt controller normally you can run higher voltage and that get converted to more charging amps.

On my 4-100 watt panels, I put each pair in series and then paralleled the two pairs. So I get half the amps but twice the voltage and my mppt controller converts the extra amps on the output side so everything is charged at full power.

Another benefit is with less input amperage you can use smaller (cheaper) wires on the long runs between the panels.

I think the Renogy controllers are rated by output battery current, so the 20 amps or 40 amp is max charging amperage to the battery, not input. At least that's the way I read the specs.

Ray I think that's way they rate the solar regulator. Not by the output of the solar panels.

Thanks for everyone's input. I now have a new Victron MPPT smart charge controller on the way.

Ray; I believe you are correct. I always thought it was the input the past 5 years. I just read your comment and looked at the spec's on my controller. I thought I'd fry the controller if I exceeded the 30amp rating on input. Upon inspection of mine, It reads, "Rated Charge Current: 30A". Which leads me to believe its the amps to the battery. So I learned something valuable today! THANK YOU.