The variable type String can cause memory-problems so you should'nt use the variable type String at all
char keybChars[4+1]; // 4 characters plus one byte for terminating zero
keyChars = "FF01";
What is not clear to me what do you want the integer to contain?
is your charsequence meant as 4bit hexadecimals or 8bit hexadecimals or do you want to convert to the ASCII-code?
the further transformation depends on that