The Nats have lost four in a row for only the 2nd time all year. Unlike the last time, a sweep at the hands of the potentially historic Cubs, these games have been against more normal competition, the playoff caliber Dodgers and the cellar dwelling Padres. It almost feels like it hasn't happened, since these have pretty much all gone in the books after midnight on the East Coast, but it has. Is it concerning? Nope.
Good teams, great teams, lose 4 in a row. The 2014 Nats, who ended up winning 96 games, went 0-4 twice. The 2012 team, which won 98 games, did it three times including a 5 game slide. It's not going to happen a lot but a couple more times this year is easily possible.
If you shouldn't worry now (and you almost certainly shouldn't) when should you? That depends on what you are worrying about.
I'm worried about making the playoffs
We are 72 games into the year and the Nats are playing at a 96/97 win pace. Basically that means that the Nats would have to play around .500 for half the year* to be in the danger zone of missing the playoffs. Regardless of how you parse the season**, that's a worse Nats team than we have seen. For me, that means we need some kind of run that would fundamentally change how I view this team. A five or six game slide is certainly not enough. I'm thinking maybe something like 0-10? 2-13? Something that drops them to that under 90 win pace. Something that makes one stop and go, "woah, maybe this isn't the 90+ win team I thought these guys were" We're a long way off from this right now.
I'm worried about winning the NL East
This one is a bit tougher because it involves other moving parts; how the Mets (and ok, we'll throw in a Marlins mention here) are doing. The Nats could go 6-5 but if the Mets go 11-0 then it's a dead heat. The Nats could go 3-8 but if the Mets go 0-11 then the NL East feels wrapped up. So there can be nothing really definitive here, unlike the playoffs where generally things end up a certain way.
We could set the bar only at what the Mets are pacing right now - which would be 87 wins. However, if you are worried about losing the East, I think you are worried about the Mets playing better rather than the Nats sinking down to finish the season with 85 wins. I think the safest bet is to make that bar a little higher than the last one, maybe like 92 wins. In this case the losing is a little less dramatic as those that would cause a playoff miss. These are streaks that would likely bring the Mets back into close contention. If you figure the Mets to be a little better than .500 than extending this streak to around 0-8 would probably do it, or something like 2-10.*** Basically the Nats have to repeat what they just did.
I'm worried about home field advantage in the playoffs
Ok if you want to be a worry-wart, here's a legitimate thing to worry about. The Cubs have slowed down a bit, but just a bit, and are still on pace to win more than 110 games. To put it another way - if the Cubs play .500 ball here on out, they'll finish 93-69. No one expects them to play that "poorly" so to have any chance to keep up with the Cubs the Nats basically can't afford to fall much off a 95+ win pace, which they are only a little bit above now.
If the Nats keep winning as they are and I'd give them a puncher's chance to overtake the Cubs, but they are on the cusp of being in real trouble. So maybe a couple more losses? 0-6? 2-8? That would likely put them 6+ more games behind the Cubs with a half-season to go. I wouldn't like those odds.
To recap, four losses in a row ain't nothing. Unless you are aiming for 100 plus wins, it's going to happen (and I'm sure some of those 100+ win teams had it happen too). The Nats have stumbled which hurts their chance at home field, but given how they, and the Mets, have played so far, it affects little else. If they continue to stumble, or worse fall flat on their face, there would be something to worry about**** but as of right now there's nothing.
*Depends where we draw the line but if we say "under 90" we're looking at something like an 83/84 win pace for 90 games.
**If you are positive about the Nats then they are who they are, a 96/97 win team. If you are negative you might say they lucked into a Braves team who couldn't get out of their own way early and really they are more like the record the team has put up since mid April. That is more of a 88/89 win team. Either way it's a long way from .500
*** Of course we are getting into a Mets series during this time frame so that's a little different but I'm talking vague generalities here.
**** Also unspoken is after the Dodgers comes the Brewers who currently are only slightly better than the Padres. The likelihood of continuing a long losing streak against them feels really unlikely. After that is the Mets, but then Cincy and Milwaukee at home. You can slip up against anyone but it's a lot less likely against teams like these. So most of this is more things to look at heading up until the stretch run rather than what could happen right now. Most. The 0-4 is here now so that counts for something.