I've been out with this guy about 4 times and spoken on the phone several times. He says he finds me attractive and says he misses me when I'm physically not around.

If he has the guts to say this, then he obviously means it.

He is humorous, very generous with compliments, well-educated, and has a good job too.

They are all very positive things.  You want a guy he matches your personality and you want an intelligent guy in a good employment position.

So far so good, but he is short (5'6") and has too many financial commitments towards his parents and sisters. It is not particularly flattering to go out with a short guy (although I'm petite myself).

I really don't see why women have a problem with short guys.  Does it make him a bad person?  What you should be looking out for, is his employment prospects because he needs to be able to support you and any children you may have, but most important of all is his personality and your enjoyment together.  Height should not come into the equation at all, nor should the size of any other part of his body.  You are very unlikely to find a perfect guy with all the features that you want, you have to compromise in one or two respects.

I can't decide if he is a good prospect for a long-term relationship.

Then you need to get to know him more, to find out.

Financial security is quite important to me.

Financial security should be important to all women, because without the finances you can't put food on the table.  But trying to find a millionaire is going a bit too far, because there are not many of them.  You need a guy with stable income and without too many financial commitments.

I'm always busy with work and don't socialise a lot. Shall I hold on to him? Any thoughts?

Yes, you should hold onto him. Get to know him a bit more, then make a decision whether you fancy him or not.