That sounds suspicious, an 8" log shouldn't even need 4 tons let alone the 11 we calculated you should have with your ram. What kind of wood?
If you have a leaky/worn hydraulic ram, that would cause it to have less power than expected.
Also, I know you said your setup is free flowing when the lever is in the neutral/middle position, are you sure its plumbed correctly?
How is the cycle time?
Take a video if you can and upload on youtube so we can see what's happening.
That 16gpm pump you have should generate plenty of psi to split the wood. Did you bleed the pump and system by cycling the ram a few times? Is the reservoir large enough? I know when my splitter runs low on oil the ram will stop short of where it needs and have 0 power when the pump cavitates.
Edit: it just looked at your pictures and that cylinder looks very small for a log splitter. I think its even smaller than you think it is. My splitter (5.5hp briggs Didier unit) has a 4" ID bore.
It looks like this: