airlock relatively quiet, but yeast cake still active in primary

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Hi all.

Brewing an English Bitter and I've had it in the primary for four days
now and fermentation has quieted significantly, but there are still
small eruptions of yeast cake in the trub, indicating that there is some
  additional fermentation going on.

I'm probably going to take a gravity reading and wait until all visible
evidence of fermentation are gone. If I find that the gravity is not
where it should be, would anyone recommend that I agitate the primary to
get the yeast back into suspension so that fermentation will be complete?

Here is my recipe:
     * Steeped .5 lbs of Maris Otter (3 Lovibond) and .25 lbs of Crystal
malt (20 Lovibond) with 1/2 tsp. gypsum in six gallons of cold water
until brought up to 155 degrees where I turned off the heat and let the
grains steep for an additional 15 minutes.
     * Sparged the grains (using a colander) with 1.5 quarts of water at
150 degrees.
     * Brought to a boil and added 3.3 lbs. of Munton's Light LME and 2
lbs. of Munton's Light DME.
     * Added 3/4 oz. of Cascade hops for the full boil and 1/2 oz.
Crystal hops for the last 15 minutes.
     * Racked to primary and poured out remaining wort through a
colander to separate the hops.
     * Created yeast starter with White Labs English Ale Yeast (WLP002).
Pitched yeast slurry into wort.
     * Ended up with 5.5 gallons of wort
     * Original gravity 1.037

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