By default pybombs runs make with the "-j4" option, allowing it to compile things in parallel. This typically helps the compile go quicker. I certainly wouldnt expect the gnuradio compile to have issues like this, but you could edit the gnuradio recipe to force it not to use the "-j4" option by adding:
make {
make
}
This basically overwrites the template pybombs uses for the make stage, which uses the "-j4" by default. It will result in a longer compile time, and again I wouldnt expect a machine with 4GB of RAM to have any problems, even with that option, but its something to try.
Tim