Advice/help needed

Hi all,

I am trying to switch to GIT (from SVN), and have become sorely confused. I am not sure of the best way to solve the problem I have, (nor how to actually implement a solution when one is chosen).

I am building a web application in php. There are 2 (soon to be four) versions each slightly different for different customers. Each needs testing when installed in "/" and "/dir" on the web site.

So far I have one system that in installed in a git repo on a Linux VM with a share. This way I can develop in windows where I am familiar, serve the files under linux (to match the production environment) and run unit tests using phpUnit. The test files are all in a sub-directory of the main directory. The application is served from a directory in the website, so I could have different directories for different purposes, but I have not had to use this yet.

Before release, I fetch and merge the files into a second installation under windows, where I can serve it from the root. This forms a second level of test. I release by copying the files with FTP (so that test files and the GIT repo don't go on the production server).

This arrangement only works because I have been able to set up the configuration files, database users and similar so they are all the same on each installation. With 4 similar versions this will no longer be possible.

What I want to be able to do is control all 4 versions in the same manner, keep all file - common, different and test - in git, and have checkout worry about changing versions.

My thoughts are to have 4 branches, one for each customer. 99% of all changes will be needed by all (or at least most) of the customers (P,W,S and E). How can I make a change to master and then use git to apply those changes to the four branches, without losing the differences between branches?

For example (if this is the best way) go from this
O-----O-----A-----B-----C  (master)
  \----P
   \---W
    \--S
     \-E

to first this, where D is the net effect of A B and C  (this is for ease 
of reading logs, and commit messages),
O-----O-----D  (head) 
  \----P
   \---W
    \--S
     \-E

and then to this, (without editing all the files four times?)
O-----O-----D  (head)
  \----P-----D'
   \---W----D''
    \--S-----D'''
     \-E-----D''''

Or would I be better having 4 repositories, one for each customer? Then I need to pull changes and cherry pick the changes I want for each customer?

I am the only developer, so the processes need to be simple so I am not faced with sorting out my own errors! :)

Regards

Ian