There are 251 4-letter words starting with B in the ENABLE dictionary — 6% of all 3,903 words at this length. B is a one-point tile, so the value in this list is positional rather than numerical — which of these you can play depends far more on where it fits the board than on what it scores.
The list divides into 9 groups by second letter, the largest being the 51 words with ba. Scanning the right group is faster than reading the whole list, which is why it is broken up below rather than run alphabetically.
If you know more than one letter, this page stops being the fast way to find it. Enter what you have in the unscrambler — the Contains, Starts with and Ends with filters combine, and setting both length fields to 4 narrows this list to the handful your letters actually allow.
All 251 words, by second letter
BA… (51)
baal baas baba babe babu baby bach back bade bads baff bags baht bail bait bake bald bale balk ball balm bals bams band bane bang bani bank bans baps barb bard bare barf bark barm barn bars base bash bask bass bast bate bath bats batt baud bawd bawl bays
BE… (33)
bead beak beam bean bear beat beau beck beds bedu beef been beep beer bees beet begs bell bels belt bema bend bene bens bent berg berm best beta beth bets bevy beys
BH… (1)
bhut
BI… (29)
bias bibb bibs bice bide bids bier biff bigs bike bile bilk bill bima bind bine bins bint bios bird birk birl birr bise bisk bite bits bitt bize
BL… (19)
blab blae blah blam blat blaw bleb bled blet blew blin blip blob bloc blot blow blub blue blur
BO… (50)
boar boas boat bobs bock bode bods body boff bogs bogy boil bola bold bole boll bolo bolt bomb bond bone bong bonk bony boob book boom boon boor boos boot bops bora bore born bort bosh bosk boss bota both bots bott bout bowl bows boxy boyo boys bozo
BR… (24)
brad brae brag bran bras brat braw bray bred bree bren brew brie brig brim brin brio bris brit broo bros brow brrr brut