Hi Daniel,
You can manually synchronize 4 AFGs to, from my experiments, nanosecond precision by tying their triggers and ref clocks together but it's laborious and error prone. On an AFG31000 you have a "multi-unit sync" menu that will take you through synchronizing two AFG31000s, but the process is largely the same for more units, no mater if they're AFG3ks or AFG31ks.
For higher precision and much better ease of use I would suggest looking at our AWG5208 which has 8 output channels, all internally synchronized.
