# [Python 3], 127 bytes

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

    def f(n):s=str(n);l=len(s)-4;return f"{round(n,~l):,}"[:4].replace(*",.").rstrip('.')+"KMBT"[l//3+(int(s[3])>4)]if n>9999else n

[Try it online!](https://repl.it/J95M/3)

[Python 3]: https://docs.python.org/3/