MB 4: This practice - Even though he tied them together, this doesn't nullify the mitzvah of tzitzis. This is true because it is not a knot that will last, since he has in mind to untie the knot immediately when he leaves the cemetery. This is like that which was written in chapter 10, paragraph 3. [This implies that if one permanently ties two tzitzios together, this would invalidate them even though they were initially tied properly onto the garment. 10:3 discussed the case where one sewed up the corners of the garment and rounded one off - but if one were to cut these stitches then the garment would again have four corners. The ruling there is that obviously he intends to remove those stitches, and therefore the garment requires tzitzis. -- YM] Furthermore, according to those who rule that tzitzis are nullified by tying them, then he is wearing a four cornered garment without tzitzis.
23:3. One who comes within 4 paces (6-8 feet) of a dead person or (5) of a grave (6) has the same law as that of entering a cemetery. (7) In places that have the custom to remove the tzitzis from the tallis of the dead while still in the house, if the pallbearers have on tzitzis, then that should cause us to be concerned about (8) "mocking the poor".
MB 5: Of a grave - Even when coming close to the coffin of a minor [child under bar mitzvah age - who was not obligated to perform the commandments even while alive], one should still be careful of mocking the poor, for perhaps it is the soul of a great person. However, no such problem exists when approaching the coffin of a woman, who during her lifetime was also exempt from this mitzvah. [We're not doing deep philosophy here, but some explanation is needed. Judaism believes in resurrection, in order for a soul to complete its purpose in the world. It is specifically the soul of a great person that might only need a few short additional years, thus dying as a child. The obvious question is that if so, shouldn't we worry even near the grave of a woman, who might have been a man in a previous life? Answer: resurrections only happen within the same gender. -YM]
MB 6: The law is like entering a cemetery: See later on in Chapter 45, in the Mishna Brurah.
MB 7: In places that have the custom: The later commentators agreed that even in a place that does not have the practice of removing the tzitzis, it is also a problem of mocking since the dead are exempt from the mitzvos.
MB 8: Mocking the poor: This is if the tzitzis are uncovered, as was explained above.