# [Python 2], 137 bytes

<!-- language-all: lang-python -->

<pre><code>def f(s):
 r,l='',0
 for c in s+'.':
&#9;if c.isalpha():l+=1
&#9;else:
&#9; if l:r+=l&lt;4 and'Blt'[:l]or'Bl'+'e'*(l-3)+'t';l=0
&#9; r+=c
 return r[:-1]
</code></pre>

[Try it online!]

[Python 2]: https://docs.python.org/2/
[Try it online!]: https://tio.run/nexus/python2#DY7BDoMgEETP@hV7WyhqNO2Jlkt/w5iGKKQkK5gF08@33GbyZjJzbc6DF1nqFrgjg9iNLfjEsEKIkBUOqNsmeFiHkC0dXyukJmWmtnGUXWVQIWlWhl4PsHHDNxWcNS2Jq0SFDm@C@rtUWPBJZqyVml7roCsnR@BZ99NyHRxiqV/Y/j4hHmcRUl6U2O0QjnzusKXqIIcCdndl@AM "Python 2 – TIO Nexus"