To get some actual numbers, I went back.

On the first hardcast, the cast time showed 1.48 seconds, this was under Leylines.

Currently, at base Spell Speed, Fire 4 takes 2.8 seconds to cast, which, if we take Ley Lines into account, it would be a 2.38 second cast.

He did hardcast Fire 4 without Ley Lines and it looked to be a 1.6second cast (though that doesn't line up with the above cast time when taking ley lines into account, but this is all rough estimate anyway).

Most importantly though, the cast always finished before the GCD finished it's cooldown.

So, it looks like they have given BLM a weave window after every Fire 4 now.