Here's a full list of all circuits in chapter 4, taken from
http://www.gamefaqs.com/xbox360/672893-dead-space-3/faqs/66089//////WARNING ! SPOILERS ! \\\\\\\
Upgrade Circuit: +1 Damage
In the same area where this chapter starts, look behind the workbench. It's
an area that is curtained. There is a slight opening, you can use Kinesis
to grab an upgrade.
Upgrade Circuit: +1 Clip
When going back to regroup with the crew, before you enter the Repair Bay,
look to your right. There will be a Storage Room 2AS. Inside this room
on the left wall will be a cabinet with the circuit upgrade.
Upgrade Circuit: +1 Damage +1 Clip
Optional Mission: Greely
Once you have balanced the power in the fuse box in the radio room, proceed
on and you will find yourself back in an area you were previously (which
now has an activate weapon bench), but is now lighted. Enemies will spawn
here. After you destroy them, search for a door that was previously
locked. This will lead to a bathroom that has an upgrade circuit on
the sink.
Upgrade Circuit: +1 Rate of Fire, +1 Damage
Optional Mission: Greely
In the same area that has the weapon bench, there will be a door that
requires a tungsten bar to open. Inside that room will be a weapon part,
Telemetry Spike. Also in this room, on a cabinet on the wall will be a
circuit upgrade.
Upgrade Circuit: +1 Reload
Once you are on the "Skip:, head over to the C.M.S. Brusilov. Fly over to
the big floating ship wreck, with the number 06 on it. On the face of the
wreck, shoot one of the decompression windows, and a weapon part should be
revealed. On the opposite side of the wreck is a door you can enter with
an artifact and an upgrade.
Upgrade Circuit: +1 Reload +1 Rate of Fire
Optional Mission: C.M.S. Brusilov
Before using the Electrical Engineering Interface to open the next area
door, look to the opposite of that Interface for a door that has a text log
and an upgrade circuit in that room.
Upgrade Circuit: +1 Damage
After docking at the C.M.S. Terra Nova, you can fly into outer space and pick
up two items in the surrounding area. There is a big platform that is
guarded by cysts, and a few mines that will track you. A storage container
is on that platform, with an upgrade circuit and a weapon part.