One of the hallmarks of great writing is when all of the storylines illustrate and reinforce the main theme. And S4E8 of the Americans was a paragon of thematic coherence.
The theme of the episode was the terrible psychological toll that spying takes on the spies themselves. One example of this theme was the subplot involving Lisa, the alcoholic Northrop worker whom Elizabeth recruited back in S2. As Lisa explained to Elizabeth, the lying and pressure of spying caused her to "crack" and resume drinking.
Perhaps the clearest example was Philip and Elizabeth's tension over Martha and EST. Yes, there were proximate sources for that conflict. As Gabriel noted, however, the real problem was the two had never had a break from spying. That was the point of the time jump -- i.e., to show the contrast between Philip and Elizabeth before and after their rejuvenating "vacation."
The point was driven home further by the contrast with Paige, who spent the last seven months spying on Pastor Tim. Unlike her parents, Paige got no such break, which was why she seemed so much more stressed and tense than the rest of the family.
As I say, it was a model of great, thematically coherent writing. Hats off to the writers.