Please note: WRFDA beginners should not use this option until you have a good understanding of the 4D-Var lateral boundary conditions control. To disable this feature, make sure var4d_lbc in namelist.input is set to false.
If you use PREPBUFR format data, set ob_format=1 in &wrfvar3 in namelist.input. Because 12UTC PREPBUFR data only includes the data from 9UTC to 15UTC, for 4D-Var you should include 18UTC PREPBUFR data as well: