"RS 232 is only 4 signals"

RS232 actually defines twenty four signals (or "Interchange Circuits") - that's why they specified the 25-way D-type!
(don't forget that RS232 covers both sync & async links)

Only two - TxD & RxD - are actually required for data transfer;

Two more - CTS & RTS - are commonly added for hardware flow control;

Various others may be added for other handshaking purposes, eg the PC 9-way D-type also has RI, DTR, DCD, and DSR.

And, of course, you need a Ground reference.