What is this prefix in TCP data?

I am currently trying to write a module that can send and receive messages to an existing system over TCP. Unfortunately, there is no documentation. The messages themselves are XML, but each message is prefixed with 8 bytes. This is a typical message as shown in Wireshark:

As you can see, the TCP packet data piece starts with 8 bytes, followed by an XML message. I noticed that the first 4 bytes of the prefix are always the same, only the last four bytes change:

04 00 00 11 [?] [?] [?] [?]

Does anyone know what this might be? I didn't recognize any patterns.