incrediBILL - 6:29 pm on Nov 6, 2011 (gmt 0)
If it's a reseller account it's probably on shared hosting, might be easier to ask them to migrate your account to a different server which would also get a new IP address but posing it a different way may get action from the host. Based on what you're saying I think it's your home IP address that's the issue, not the server IP address since others can see the PHP running, so doing this might just be a waste of time. Especially if it's something tweaked wrong in the hosting account, which I'm thinking since it's causing a 404 just for PHP files and just to your home IP, you may end up simply moving the problem to a new server.
Was there anything in the software you were using that tracks the IPs being accessed that give allow/deny access you might have inadvertently enabled?
Perhaps clearing out your temp folder on the server will clear out the problem, I would try this as well.
Also try wiping out all cookies in your browser related to your accounts, clean 'em out, sometimes this can be an issue as well if something rogue gets stuck in your local cookies.
Worse case, since you can't seem to get a new IP from your ISP, a quick fix would be to set your browser to use a proxy server IP so you can still continue working until the problem is resolved, if ever! ;)
BTW, if you put the same PHP files where they can be referenced under a domain name on the server, do they then work from your home IP?