# [Python 2], 132 bytes

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

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

[Try it online!]

[Python 2]: https://docs.python.org/2/
[Try it online!]: https://tio.run/nexus/python2#HU7LCsMwDDu3X@Gbk6ULHdspWy/7ja6M0KYs4D5wUvb5nTuwjGQJrH0II4wqaVcCV9QgVnUJ48LQQ5whGXzV6MoijtDbmDytH6@0I9NcyiJQCuKBmOTYNAqfhAYDnhSdr9pgxkpOGVtHnW7pceskLcH@To284ZA3noH3leOcpQb77zvO65aV1jstHCaIa9omGBZRkGIGP4VswYM/5g/ZMWM6qLX20D8 "Python 2 – TIO Nexus"