Camp 4 in mid March?

Planning our first valley trip. All the info I've found talks about how busy things are in the spring and how empty it is in the winter, curious about the middle ground. Will there likely be spots available? 

It will likely depend on weather. That being said, even if camp 4 is a zoo, you can still make it work.

thanks trevor.
On the day that we get to the valley we wont get there until like 4 in the afternoon. I read about having to be at the ranger station early in the morning, do I need to get a hotel for the first night or something or do you think it'll work out?

If it's like last year in mid-March and it's snowing everyday, Camp 4 will be empty

March can be amazing depending on the weather and what your objectives are.  Temps can be great for free climbing but getting an extended weather window for a route on the captain can be a gamble.  Sometimes is great and other times you could get significant snow.