from serial port each and every letter or digit is received as ASCII value that is also saparately
that means if you want to send 4 digit value as 3425 then serial port will send 4 ASCII values
33h, 34h, 32h and 35h
you have to store all four values in an array (or table)
then you have to write a program to convert all 4 ASCII values into digits and finally combine them into single value