Keith,
Welcome to the Forum!
You didn't indicate the weight of your slab. For "most" the 4 hours @ 225 would have done it...but it didn't. My guess is that your slab probably only needed 30 to 45 min more. One slab at 2 1/4 pounds will be done almost an hour faster than one that's 3 lbs.
If they were tough, they needed to smoke longer. Stuart's idea, the foil, is good to keep them from drying out.
Try this. At the three hour point, check them. If they're not done, pull them out, wrap them in foil. At this point you can also spray them with a little apple juice if you wish.
Remember what Smokin' says...
It's done when it's done which for you means, Check them before you pull them out to eat.
Try this also, "shake" a bone to see if it's as tender as you want. If it's not, then let it go 30 min more, and so on until you're done.
And of course, you DID read the Ribs 101