If you are looking to create an "int" based on the value of four "byte"-s, then you may have to start using the shift and boolean operators. This is getting into more advances stuff, but look at this example:
In the first part you have the numeric values and you want the new int to be the result of the four bytes. The "&" does a logical AND of the bits; the "<<" shifts them; and the "|" ORs the results. The end result is that you get a 1 in the first bit, 2 in the second bit, 3 in the third bit, and 4 in the fourth bit. Which gives you the number 16909060.
In the second example you have the String "1234" broken into four bytes and you are converting it, which leads to the number 1234.
So the answer to your question is, it depends on what you are trying to do.
With your second questions, if you google on "Java primitive" you'll find out a lot of information on
Java primitives. "byte" is an 8-bit signed values (-128 to 127). "char" is an unsigned 16-bit value (0 to 65535?). "int" is a signed 32-bit value. String is an object (not a primitive) that happens to be based on Unicode.