The sus 4th note is related to the chord, which is Bm7, as opposed to the key.

The melody goes F#-E-C#-D-E-C# (approximately), so you can start it with either a D chord or a Bm7 chord, but you can't start on the Bm7sus, which you can only use as a passing chord over the E note, and you have to start the phrase on either a D or a Bm7. It is probably better to call this chord an extension rather that a suspended chord, since you really play a Bm7 and extend it to add the E before you change chords.

You may note that it is possible to substitute the Bm7 for both the D chord and the E chord. This is works out for the reason that Bm7 is the same as D6, and at the same time it is a part of the eominant hord, E9. A neat little trick that let's you turn it into from a three chord song into a two chord song. It is technically simple, but theoretically complex.

Just to let you know what an obsessive-compulsive I am, I worked up a chord/melody arrangement of this tune(admittedly not a particularly long or difficult tune, but,,,) purely for the purpose of finding an answer to this question.