Since there aren't any other credits..I'll add Bruce Jones to all 4.
I would advise against this. "Art by" is a tricky credit. The only thing you can really say for sure is that it means he or she did the pencils. Frequently, it also means "inks", but you would need to establish that they have a track record of doing inks in other publications before, I think, you could safely say that. If you're at all in doubt, I'd put them down for inker, but also used the "(unconfirmed creator)" credit.
Lettering is hugely tricky, I think. This was a quite under-credited position throughout the years. Very early on, in the America of the 30s, often it was actually the artist who lettered. But the percentage of comic books from the late Golden Age to the present day where the letter is the penciller is very, very, very low. The fact is we may never know for sure who did the lettering on a lot of books in the middle of the 20th century, because back then it was viewed as a technical, and not artistic, position.
Colorists are even harder to predict. It is an even rarer accomplishment for the colorist to be the penciller. These days, it basically only happens when a work is painted, as opposed to drawn. Or, when the coloring job is relatively unsophisticated, as with (American) comic strips. Honestly unless the work were painted or the artist was someone like Aragonés, I just would set it as "unknown" and live with the fact that we'll probably never know.
In any case, we would prefer that you not leave creators blank in a case like this, but actually take the time to use the creator, "(unknown creator)" (parentheses necessary). This shows that the work in question is as completely credited as possible, rather than just incomplete. It also gives us a handy list of all titles where some additional sleuthing may be possible later. Think of it as a way to say, "I tried my best, but this is as much as I could find at the present time".