Originally Posted by UselessBettor

If you find no other horses from that race where this horse won have gone on to be competitive or win then yes it is a weak field and you discount the winning lengths, but if 2 or 3 of those horses have gone on to place or win other races then its a very good 4L win.

As always its a "depends" which make the difference of a profit or a loss and it needs to be coded into your system logic or handicapping process.