Set variable value with serial?
You are comparing a byte which was read through uart with a number. If you send number '5' through uart, you won't get number 5, but character '5'. To convert single character to number, you can do following:

Luku4 = Serial.read(); // '0' .. '9'
Luku4 = Luku4 - '0'; // 0..9


But this will work only for single digit numbers, for more robust solution please refer to readString function to see how you can read strings from UART and to function atoi which converts string to number.