Why not try the hp5+ dev times as a starting point. Your hp4 will have lost some speed and contrast. I'd be tempted to rate it at 200-320 or so and develop normally for 400. Why not expose a sheet and snip it up and develop each to different times and find your own answer...
You should probably develop(<---fixed mistake) an unexposed sheet and have a look at the fog before putting time into it, but it's probably okay. I have shot 40 year old panatomic x at box speed and developed it normally and gotten good results.
I use ID11 1+1 most of the time for fp4+ and hp5+, but you can find times for hc-110 on the massive dev chart.
Well, I tried this tonight. With carefully measured temp and time, and thorough fix and wash, there are substantial fog issues with this box of film. (no surprise there) When I get a chance, I'll mix up some of the expired film developers to see what I can salvage.
Tom, on Point Pelee, Canada
Ansel Adams had the Zone System... I'm working on the points system. First I points it here, and then I points it there...