How to fix sagging sub floors?

No. putting a 2x4 across the floor joist will not help. I seriously doubt a house built in the 70's would have joist spaced 48'' OC, possibly 24'' but not 48". If that was the case the problem your bigger problem would be the floor sagging between joist which would not be helped at all by the 2x4.. If by some chance you are right about the spacing what you will need to do if fill in the missing joist with more joist of the same size by adding 1 or 2 in between existing joist to bring it to 24 or 16'' OC. If the joist themselves are sagging, which is what I suspect you could add a 4x4 or larger beam across the existing joist and support it every few feet on concrete blocks. This is uming that you have a crawl space and not a basement. If you have a basement you may need to go with something like a 6x10 or 6x12 supported on columns. I doubt this would be worth the effort if there is only a 1/4 inch sag.