Connect 8051 serial port to multiple 8051s
I wouldn't imagine fan-out or power problems so connecting the TX line of the master to four RX lines should be fine. The problem is in the reverse direction, you'll be connecting four TX lines (that are all outputs) to the master's RX line. A couple of solutions come to mind:


Enable the UART receiver on all slave devices but leave the transmit line in a high-impedance state. When a slave has received a message addressed to it set the transmit line as an output, send the message and then go back to a high-impedance state. Some microcontrollers support doing this and some don't because enabling the UART sets TX as an output an RX as an input automatically. If you use this method you'll need a pull-up on the master receive line so it's not floating when no slaves are transmitting.
Use a 4-1 multiplexer on the receive line of the master so that before sending a command to a slave it can select one of the slave TX lines so only one is logically connected at a time.


Option 1 is the way I'd go if using a microcontroller where the UART receiver can be left enabled while the transmitter is disabled. Other options to consider would be SPI and I2C which are both designed for multi-drop communications. Serial (which you can convert to RS232) is probably a nice option for a student project though because of the ease of debugging communications with a PC.