4GB RAM is still not enough in some cases. For example, I have an 8GB RAM sandbox with virtually no traffic but is a multisite setup to allow for having 5 disposable sandboxes. Rebuilding today failed due to Error 137 (OOM) and I had tried the trick @richard suggested above. However, to save myself from the hassle of doing this everytime, I’ve created a larger swap (4GB) which seems to have allowed the rebuilds to happen for the time being. It seems like we’re just upgrading servers in the last 1 year because discourse rebuilds are really getting RAM hungry for some reason.