I think it might be because he's overtired by the time bedtime rolls around. Have you tried to extend his naps?
The average A time for a 4.5 month old is about 2 hours. Have you tried scaling back his A times from 2.5 hours? Or, if he's on the high side of A time, have you tried 2.25 hours (that's where my DS is right now at almost 5 months). A time is from when he wakes up to when he goes down for a nap.
I think a combination of tweaking the A times and extending his naps will help with the night wakings (and everything else in general). Also, this should help with extending those feeds, too, as formula fed babies normally can go 4 hours between feeds. I know that when we were having problems with DS's naps being short, he would wake up cranky and fussy and wanted to soothe himself with eating (though he wasn't *really* hungry).
If none of that sounds right to you, then it possibly could be the DF messing with his biorhythm. Have you tried dropping it and just letting him wake up during the night when he's hungry (really shouldn't be more than, say, once after bedtime)?