UPS Recommendation for ReadyNAS 204

A SMART-UPS with a USB port

How big if I only care about powering the NAS with 4-drives (2 right now)??

Don't need that much runtime to shut the NAS down safely. Allowing a minute or two for the NAS to shutdown would be plenty. Also, you don't want to put more than 80% load on the UPS or else the battery will drain too quickly.

So it depends on how much runtime you want which model you get.

2 minutes is fine.

be nice to have a number ...

APC has a product selector tool on their website.
The entry level units are fine but have a year less warranty.
Smart UPS are not cheap compared to the Back-UPS range. These cheaper ones with USB monitoring have RJ50 to USB proprietary cables which can be problematic and are expensive to replace. The smart ups units will accept a standard device to USB cable.
There are other manufacturers of UPS devices.

I doubt you'll get a specific device number from anyone, it depends on where you live, prices locally, your current and future load, warranty requirements.

I also believe it helps do do your own research, you have a better understanding of what you are buying.

Personally I use a CyberPower CP1500AVRLCD with my RN202.

It is shared with a nearby desktop PC, but the USB connector is attached to the RN202.

I have two other CyberPower in the basement with the main network stuff - they protect several NAS (including an RN100), my main switch and router.