If you look at page 73 in the BRB, it stats the rules for dreadnought close combat weapons.
It stats that for each DCCW the walker has after the first one, it gets and extra attack. And do the the fact that it DCCW got their own special rule about extra attacks for more extra cc weapons, I would think you shouldn't look at the normal rules for having and additional cc weapon.
So to explain your 2 cases above:
dread with 2 dccw's would have 3 extra attacks as standard (2 standard + 1 for an extra dccw)
dread with 4 dccw's would have 5 extra attacks as standard (2 standard + 3 for an extra dccw)
Hope it helps;)
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