- They left at six am and would reach London after four hours.
This states that they did leave at 6am and did in fact arrive in London after four hours. However, the statement can be read so as to place the reader at a point in time after departure but before arrival. You could paraphrase the above to read (albeit less elegantly);
They left at six am. We know now that they arrived in London four hours later.
The statement about the time of departure is part of the narrative, but the arrival time is a point of fact highlighted to the reader, but out of context of the timeline in which the story is told.
- They left at six am and reached London after four hours.
This states that they did leave at 6am and also that they arrived 4 hours later.
The book, Oxford Guide to English Grammar by John Eastwood, says
'would' can be used as the past form of 'will'. Is it always the case?
Yes, I can't think of an exception to this, but there are of course other uses of the word would.