not full answers, but clues to the answers and some observations.
1. from looking at your stats you only have 3 hosts which have returned credits. i would check on the fourth PC.
are your computers really running 24/7?
check your preferences in your accounts to see if boinc is running while other applications are running?
On your account page in seti, view computers, click on any computer name and check the "% of time boinc is running" at the bottom of the page. mine is 98.6% and "while Boinc is running % of time work is allowed" mine 99.98%.
What % are you getting?
You haven't scored any credits on einstein since mid january.
Your seti credits per day are good, but up and down. scores of 0 to 524 over the last ten days. my RAC is 85 with 9 projects running
check the websites find out how the RAC is calculated in detail
2. you are using boinc since Nov 2005.
Credits are calculated in the following way.
the wokunit is sent 3 or 4 hosts. all hosts return a value. An average of all results returned or 2nd or 3rd returned wu score is the granted credit. check the boards on the websites for more details.
I hope this is of some help.
bjango