Network Performance - 4 Computers Sharing An Access Database?

I am working on machines at a local car lot heres the system details.Verizon DSL wireless router, don't know the exact model but all 4 computers are hard wired to it with newer cat5e.4 newer desktop computers, within months. all 4 same specs. They are HP systems with 8gb of ram running Win 7 x64 They use a program called car lot manager, it basically is installed on 1 machine and creates an access database. Then that folder is shared. The other 3 computers install the software and access the database file through that share.The database file was about 100mb which I figured was probably the issue, so I cleaned it up and compressed it. Its now down to around 40mb and has the exact same issue.-- The issue is, if more than 2 computers have the car lot manager open (not even using, just open) it begins to move extremely slow to where windows take 2-3 minutes to open and the application hangs. And I'll add we have tried different combinations of which computers are running the software, trying to see if maybe 1 certain machine was the issue.I have been on the phone several times with the programmer of the software and he claims it is ran at several large car lots as well with 10 or much more computer systems and he verified everything is setup correct and looked through his software.The original setup was the Verizon modem into a netgear 8 port gig switch, then the 4 computers into it. I removed the switch just to see if anything changed, it didn't seem to make a difference.