My guess would be that this is caused by memory leak. There is several bugs in 5.8.1 version so please as first step upgrade to latest 5.8 version 5.8.4. NGFW 5.8.2 and 5.8.3 has several fixes and 5.8.4 has updated OpenSSL libraries to fix the latest OpenSSL vulnerability CVE-2015-1793.
to be precise the memory usage is not 99% and the usage looks normal. If you look more closely, buffers and cache consume about 2GB of RAM:
MemTotal: 3951044 kB
MemFree: 41884 kB
Buffers: 187872 kB
Cached: 1829612 kB
As you can see cache is using up most of the space. The Linux kernel borrows unused memory for disk caching, and if any process wants more memory it will just be taken from the cache immediately. Buffers can also be discarded to some extent if needed so both of these types of memory can be considered available. In fact, if the buffers and cache are close to zero then you've likely got a memory problem as it indicates there is no unused memory to borrow for caching. In such case you'd likely also see swap being used heavily, which is another clear sign of memory issues.
So in practice you've got MemFree + Buffers + Cached available: 41884 + 187872 + 1829612 = 2059368 (2GB). The other 2GB is used by sg_inspection as indicated by "top", and this is not abnormal in inspection-heavy roles like L2FW and IPS (or even FW/VPN if you inspect all traffic).
I recommend using the "free" command to monitor memory instead, in below example you can see on the second line (-/+ buffers/cache) that by subtracting buffers/cache you've actually got 1245116 kB free. So that's the number to look at instead of the free amount on first line.
total used free shared buffers cached
Mem: 1984292 1690620 293672 0 96044 855400
-/+ buffers/cache: 739176 1245116
Swap: 499704 632 499072
If there was a severe memory leak you would see more memory being used than what is reported by processes in top. For example if you have 90% usage even though when counting together mem usage of all processes it only adds up to 50%, i.e. more memory is allocated than what is claimed by processes.
Anyway despite this I would recommend upgrade to 5.8.4 like Tero said, there are lot of fixes.