RAM upgrade not take in account

[bastet]

My computer is a Acer Aspire 5735

The initial RAM was 2.9GO: 1 stick of 2GO and 1 of 1GO.
I substitute the 1GO stick by a 2GO.
The problem is that it isn't taken in count in the system monitor, and ubuntu doesn't handel the 4GO (it uses swap when needs more than 2.9GO).
System monitor should show 4GO, but still shows only 2.9GO.

And free -h, returns:

              total utilisé libre partagé tamp/cache disponible
Mem: [b]2,9G[/b] 923M 1,0G 187M 964M 1,6G

Partition d'échange: 5,2G 0B 5,2G

lshw -c returns :
sudo lshw -c memory
 *-memory
       description: Mémoire Système
       identifiant matériel: 15
       emplacement: Carte mère
       [b]taille: 4GiB[/b]
     *-bank:0
          description: SODIMM DDR2 Synchrone 667 MHz (1,5 ns)
          produit: SODIMM000
          fabriquant: Mfg 0
          identifiant matériel: 0
          numéro de série: 1234-B0
          emplacement: M1
        [b] taille: 2GiB[/b]
          bits: 64 bits
          horloge: 667MHz (1.5ns)
     *-bank:1
          description: SODIMM DDR2 Synchrone 667 MHz (1,5 ns)
          produit: SODIMM001
          fabriquant: Mfg 1
          identifiant matériel: 1
          numéro de série: 1234-B1
          emplacement: M2
        [b] taille: 2GiB[/b]
          bits: 64 bits
          horloge: 667MHz (1.5ns)

In the bios, the 4GO appears.

sudo dmidecode | grep -i "maximum" returns:
[sudo] Mot de passe de principal :
 Maximum Size: 64 kB
 Maximum Size: 4096 kB
 Maximum Capacity: 4 GB
 Maximum Error: 100%
 Maximum Value: Unknown
 Maximum Value: Unknown
 Maximum Value: Unknown

The problem is the same from a live version.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: gnome-system-monitor 3.18.2-1
ProcVersionSignature: Ubuntu 4.4.0-34.53-generic 4.4.15
Uname: Linux 4.4.0-34-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Aug 26 16:09:12 2016
ExecutablePath: /usr/bin/gnome-system-monitor
InstallationDate: Installed on 2016-05-31 (86 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
JournalErrors:
 Error: command ['journalctl', '-b', '--priority=warning', '--lines=1000'] failed with exit code 1: Hint: You are currently not seeing messages from other users and the system.
       Users in the 'systemd-journal' group can see all messages. Pass -q to
       turn off this notice.
 No journal files were opened due to insufficient permissions.
ProcEnviron:
 LANGUAGE=fr_LU:fr
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_LU.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-system-monitor
UpgradeStatus: No upgrade log present (probably fresh install)