With the setting at 4 days, how many WUs does your cache carry ?
For me, I had to specify the number of WUs that I wanted because it would never fill the cache with a full 4 days worth.

Also, there are now many proxy servers, both Dnet supported and Team supported, so there is generally very little change of the Dnet WUs being unavailable. But the link between your ISP and your grid is the most susceptible to downtime.

I generally carry a cache of 100 RC5 WUs and 40 OGR-P2 WUs on each client. When it runs out of those it defaults to generating random RC5 WUs which are just as productive. The client updates the range not yet covered so that the generator does not go back over WUs that have already been done as part of the regular WU production.

So in reality, running out of WUs is not a huge concern unless you have turned RC5 off and are only running OGR-P2.