Understanding astable multivibrators
"R1 and R4 are low value resistors, am I right in thinking they just limit the current going into the transistors? Do they have any other purpose?"

It is a common misunderstanding that these collector resistors would have the task to "limit" the collector current. However, the BJT always acts as a controlled current source - and the collector current is determined by the base-emitter voltage only. It is the only purpose of these resistors to create/allow  a voltage drop at the collector node. Otherwise the collector would have a fixed dc voltage +V.