My 4 month old didn't keep much of his feedings down today-spit up a few times. I'm worried about dehydration. Is there a way to know if he is OK? Should I let him sleep through the night or check him? He had no fever when he went to bed but was fussy today.

Often it can appear as though they are spitting up much of their feeding(s). Some babies do this on a pretty regular basis.

A rule of thumb is that if they do not appear to be in pain or ill, eating normally, their behavior is not unusual, things are likely to be fine.

For the peace of mind, check on him a couple of times. When he feeds again, try a smaller feed, and see how he does. Over the next day or so, try smaller but more frequent feeds, burping well if needed, have him upright for a bit after each feed before lying him down.

Certainly if you continue to be concerned, give his doctor a call.