Perhaps if you give us a little more information we can help you out more.  If
you take an atmel or scenix micro you could fairly easily develop 4 software
UARTs if they are all running at the same speed.  The PIC probably wouldn't run
fast enough unless you are doing very little or no processing of the data.  (I
just mashed a 4800bps full-duplex software UART into a 16c54 running at 4MHz
with a small bit of data processing...  It took a /lot/ of planning...)

It is unlikely that you will find an 8-bit micro with 4 hardware UARTs (as a
hobbyist), so you probably should plan on finding one with one or two UARTs and
build the rest in software.  If I were in your shoes, I would build them all in
software.  If you run a scenix or atmel at 24 MIPs, you'll have exactly 768
instructions (scenix, varies for atmel) per bit time to run the UARTs and your
user process (at the specified 31.25Kbps).  Divide it by four (bit slicing) and
you end up with 192 instructions to read the state of four RX lines, change the
state of four TX lines, and then do some data processing.  It really would not
be too difficult.

-Adam

Edson Brusque wrote:
>
> Hello,
>
>     someone knows if there's a microcontroller with 4 USARTs capable of
> working at 31250bps? I need extra 24 I/Os minimum. One USB port would be a
> plus. Also, I would need an A/D converter, but I could use an external one.
>
>     Maybe a SCENIX can do 4 USARTs as virtual peripherals???
>
>     Best regards,
>
>     Brusque