You can indeed do this straightforwardly, and from System 7.6.1 on, so it'll be no problem on the pb190. You will need an updated Appleshare client (dating from the era of System 8.6, I think), an ethernet card, and possibly an updated Open Transport. It all depends on what you've already got on there, but I can tell you definitely that a stock 7.6.1 install and (as I recall) a standard 8.1 install won't do the trick. You will need the updated files.
There is a write-up on how to do it, and links to the required files, rather elegantly provided by Dan Palka over on
www.system7today.com , so I need neither repeat the information nor take any credit for it. I network a couple of Quadras running System 7.6.1 to my G5 Imac running x.4.9 this way, and it doesn't skip a beat. I presume that the Intel switch could not affect the solution, given that the protocols are one and the same, but as I am not running Intel Macs, I have no way of knowing first hand.
It will not work the other way, mind, in that you couldn't mount the share from your X.4 machine (unless you use Shareway IP, anyway), but only from the pb190.
http://main.system7today.com/articles/tutorials/osxfilesharing.html