I wasn't sure what to write today, so Mike Sando comes to the rescue.
Seattle would prefer to keep five receivers active on game days. The table might show why. The Seahawks' offense scored 10.5 additional points per game when the team kept a fifth receiver active for games last season (based on information available in official NFL gamebooks). I can't prove there's correlation here, but the team did average more than two additional passing first downs per game with the extra receiver available.
I don't see how having an extra receiver would so dramatically affect scoring and I imagine this is just data molestation, but let's see. Let's try to clean this up a little. The first thing that must be removed is points created by the defense and special teams, neither of which is influenced by an additional receiver. Next, since bend but don't break defenses exist, but are rare and often don't carry year to year, let's compare yards rather than points. That should further remove the impact of good field position created by the defense and special teams. Then we'll adjust for pace, finding yards per drive. Finally, we'll use drive stats to create an opponent adjustment. So what are we left with?
9 games with 4 WRs
Avg: 322.22 yards
Outlier: @ Pittsburg: 144 yards
Avg w/o Outlier: 344.50
7 Games with 5 WRs
Avg: 384.43
Outlier: @ Atlanta: 501 yards
Avg w/o Outlier: 365.00
You don't have to remove the outlier, but I do think it's instructive. I refer to the Pittsburgh contest as an outlier because it's more than two standard deviations (71.07 yards) from the average of all nine contests. I refer to the Atlanta contest as an outlier because it was week 17, the Falcons were in ruins, Seattle was resting many of its starters and nothing within that game represented reality. Now let's adjust for pace. I tallied the total number of drives, removed punt and kick return TDs and removed end of half kneel downs.
9 games with 4 WRs
Yards Per Drive: 27.88
YPD w/o Outlier: 29.32
7 games with 5 WRs
YPD: 31.66
YPD w/o Outlier: 30.00
Even more than before, most of the difference lies in two games, Pittsburgh and Atlanta. But we're not done, let's now adjust for opponent.
9 games with 4 WRs
Average YPD: 28.21
w/o Outlier: 28.58
Percentage of Average YPD: 99%
Percentage of Average YPD w/o Outlier: 103%
7 games with 5 WRs
Average YPD : 29.00
w/o Outlier: 28.65
Percentage of Average YPD: 109%
Percentage of Average YPD w/o Outlier: 105%
So, removing the impact of defense and special teams, and adjusting for opponent and pace, Seattle did perform substantially better with five active wide receivers than it did with four. That difference, though, is almost entirely comprised of a 144 yard disaster in Pittsburgh and a week 17, 501 yard free for all against Atlanta. The seven games featuring five active wide receivers also encompass the three games Shaun Alexander missed with injury. Further, no adjustment has been made for the quality of receiver lost or the quality of that receiver's replacement. Notably, D.J. Hackett had but one reception for seven yards in the nine weeks Seattle activated four receivers, but 31 receptions for 377 yards in the seven weeks Seattle activated five receivers.
All that data rigmarole can be boiled down to something simple: I don't expect the number of wide receivers active on Sunday to have any impact on Seattle's offense.