# [Python 3], 107 bytes

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

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

[Try it online!](https://repl.it/J95M/2)

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