In .net, I need to securely transfer files 4mb in size from client machines running my software to a central server, process, and securely send back results consisting of a pdf file, a jpg image, and xml data.
What .net technology would be most appropriate for this? (web service. remoting, etc)
WCF- If it is .NET to .NET (like it sounds like it is), then you should use WCF with TCPBinding. There are lots of good examples here to get you started. You might want to consider streaming if scalability is a concern.
If you want to host in IIS (then you can't use TCP- but consider WAS), make sure you use MTOM. If you're still unsure as to which binding to use, then have a look at this binding decision diagram.
WCF has a lots of security options for you to consider. You will find the Patterns and Practises- WCF Security Guidance useful for this.
Please note that ASMX Webservices and remoting are now considered legacy- WCF is the way forward.
Update- If your operating system supports it, then you may wish to consider using WAS.