How big and complex is your map?
Do you have many other programs running at the same time?
Are you on Vista?
:P I am here, and it has died a memory death a couple of times. Mainly due to a program error, I think, but if you have Vista + a lot of programs running that might eat up the memory.
You can easily run out of memory compiling even if you have 4 GB of RAM. With a big enough map it could easily take 100 GB to compile. Hint: HL just isn't for making big maps.
But: there are ways to make the HLRAD use less memory, but it will slow down the compile, or make your map look worse. Here are your options:
hlrad -chop 128 yourmapname This will make your lighting 2 times less detailed and take up 1/4 as much memory. It's also faster.
hlrad -chop 256 yourmapname This will make your lighting 4 times less detailed and take up 1/16 as much memory. It's a lot faster but looks crappy.
You can crank chop up even higher, like 1024. That is only needed if you have a HUGE (like maximum sized map) with crap everywhere.
hlrad -sparse yourmapname sparse causes hlrad to use a lot less memory but it is slower.
hlrad -nomatrix yourmapname nomatrix causes hlrad to use basically NO memory, but it has to do a lot of the same calculations over and over so it's a LOT slower for most maps.
GOOD for multi-core processors. BAD if you have a lot of opaque entities!