Any UART with USB to plug into the Nano, and with specs compatible with the device you are communicating with is good. What voltage level does your device use, e.g., 3.3V? 1.8V? 5V?
As examples, there are a lot of cables here which are FTDI, but differ in voltage and connector options (e.g., loose/bare wires, 6-pin, individual wires with pins, so on):
(note that you only need RS-232, the more expensive RS-485 are not likely to do what you want unless your devices support RS-485, and most do not)
Notice that some are 3-wire, which is ground and TX/RX. Others are 6-wire, and this adds CTS/RTS flow control (there is also a power related wire, but normally you wouldn’t use this). If you don’t need CTS/RTS the 3-wire would be a better choice, but 6-wire still works.
The problem I tend to see with multi-port adapters is that they want to use a DB-9 connector, and I assume your devices do not use this unless it is something old. I am guessing that using a USB HUB and four individual USB-to-serial adapters is not what you want to do, but that would be a simple solution, and if the HUB is externally powered, then you could remove its power consumption from what the Nano uses (this would be more stable).
I doubt you want to build your own board, but I’ll throw the idea out there anyway: FTDI chips are available where you could put your own USB HUB chip and four FTDI serial USB UART chips on a board and get exactly what you want, e.g.:
Possibly there are some m.2 boards out there, but the ones I am aware of use the higher voltage/larger connector DB-9 connector.