Objective Development Forums

I'm looking to build an AVRDoper programmer, but before I do, I'd like to understand (at least generally) what each part of the circuit does.
I've come unstuck with D4 and R15. Can anyone give me a rough summary of what they do?

Many thanks in advance!

They ensure that we have a pull-up to Vcc on RESET if we don't apply a high voltage to RESET.