How to convert bytes to long...

I think it would be easer to explain what it is I am trying to do rather than just ask a question. I would like to write a program that compresses files and then saves them into a temp. file. This file would contain all the compressed data. I have started to do this as follows:

1)Load the file to be compressed into a byte array
2)compress the byte array with zlib
3)find the origanal and new size of the byte array*
4)get the file name of the compressed file
5)now I start saveing the data into one file
    first I save one byte telling how many bytes are in the compressed files name, next I save that file name. Then I save the origanal and the new size of the byte array as longs(4 bytes each) Then I save save the actual byte array. This is the easy part. (The part I have done) Now I need to read back the file...

First I get the entire file and put it into a byte array. I then use the first byte to tell me how many bytes to read for the file name. Then next 8 bytes(heres the problem) contain the origanal and new(compressed) sizes of the byte array. They are both longs I get these 8 bytes and I don't know how to convert them back into the long long format. To simplafy the problem. Use the put command to put a long number to a file then load that same file into a byte array. This byte array should only be 4 bytes long. I then need to convert those 4 bytes into the origanl long. Wow big... Hope some one can help! Thanks in advance!!!!