Update debian bootstrap link to bootstrap4

5 jobs from master in 8 minutes and 18 seconds (queued for 12 seconds)
Status Job ID Name Coverage
  Build
passed #9161
build_bionic

00:03:32

passed #9160
build_stretch

00:03:49

 
  Sign
passed #9162
sign

00:00:28

 
  Deploy
passed #9163
pages

00:00:27

passed pages:deploy

00:00:02