Watches: 3 vs 4

The gemarrah in brochos 3b tries to determine how many watches there are in the night.

the motivation is that the time limit for the evening shema, according to R. Eliezer, is the end of the first watch. The number of watches will, therefore, determine when this time limit arrives.

What are the points:

1. The number of watches never changes. Gideon and David and we have the same system of watches. In our day, we might doubt this, but the gemarrah does not.
2. the language of Tanach is absolutely precise. If it says watch, it means one, watches, more than one ( presumably 2)