My concern is not with your gear, or your water weight; others will help with that. My question is, how old is your dad?

Now, I'm not going to try to discourage you from your trip, and I'm going to freely admit that I know nothing about hiking in Grand Canyon - except that I don't think you've got a convenient, trip-shortening bail-out point. As I understand it, your only options are, up to the halfway mark, turn back; beyond that point, keep going.

What kind of pace does 4 days lock you into for this hike? Are you confident of your abilities to maintain that pace for 4 days? For example, if it's a 15-mile-a-day pace, I'd be asking whether you've left yourself a safety margin; if it's a 6-mile-a-day pace, I'd be more comfortable. You can always slow down and kill some time along the trail, but if the minimum pace is too much, how will you make up any time you lose because you can't keep up the pace?

I think you definitely should take at least one two or three day trip before you go, maintaining the same pace you'll have to use on the trip, to get a feel for your capabilities versus the demands of the Canyon.

Perhaps others can fill in details for you on whether 4 days is too aggressive.