2 refers to 'the boy bringing the milk'. He must be bringing it quite a long way, as it has been a long enough journey for him to have been ill on the way! (Maybe he had a hangover.)
4 is even odder. It says that the tile shattered spontaneously in mid-air. (Perhaps this is a new urban sport, a bit like clay-pigeon shooting...) If you wanted to make it clear that the shattering happened after the falling, you would have had to use a past participle: 'A tile, having fallen from a roof, shattered into fragments.' And this defeats the object of reducing the relative clause - "having fallen" isn't shorter than "which fell".
(Here I am not being entirely serious: people do say 'A tile, falling from a roof...' when they mean 'A tile, having fallen from a roof...'. I wish they wouldn't though. And, without the commas, a_tile_falling_from_a_roof is airborne.)
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