If you expose at "box" speed (meaning the film box, not the Diafine box) I'll bet you don't like the negatives. Try it and see - you might. I understand what Leigh is saying, and people have argued endlessly over how "real" the Diafine speed gain is (i.e. shadow speed) but the negs look overexposed and IMHO don't print well like that, at least 35mm negatives. I have used Diafine for 120 with good results. As someone mentioned, it's great for taming Pan F+ making that one of my new favorite combos in 120 for bright conditions. But I've not tried it for 4x5.
Actually, I'll partially retract the above. Some films get little or no effective speed gain in Diafine and will look fine at film box speed - most t-grain films for example don't do what, say, Tri-X does. TXT is almost made for Diafine. Great combo, but I bet you won't like it at 400. Anything from 800-1600 will look better depending on your metering habits and the light, but don't take my word for it, just try it. You won't know what it does for you until you do.