Is it best to alter a dress that is to small or get a bigger dress and make it smaller?

A bit snug can be altered by letting out the seams. A quarter inch on each side seam will give you an extra inch.

Altering a bigger dress is easier because you have more fabric to work with. To alter a smaller dress, you need to be sure the fabric allowance in the seams is big enough. Too many off-the-rack dresses like that don't have much of a seam allowance and you won't be able to let it out enough. You will also have to find fabric that matches and a way to make the patches invisible. Sorry to say! Maybe someone else can wear the smaller dress, or your cousin will reimburse you for having to buy a second one?

Also, you can do alterations on a bigger dress which can be reversed or let out as your daughter grows, in case you want to keep the dress for "good" until she outgrows it. For example, a narrow sash can snug it in for now, get let out later.

You may not be able to get a new dress in time, unless it is off the rack. If the smaller dress has enough seam allowance you should be able to make the necessary alterations without the expense of buying a second dress. Also, if your daughter works at it she can loose 5 pounds safely between now and the 20th.

Well if you find a really good alteration person then they can work their magic. Also if the dress was really long then they could take some off of that. I would say find an alteration business and ask them if it is possible.

Making a bigger dress smaller is MUCH easier and will look much better

if you make a smaller one bigger you will have a perforated like where the old stitches were and it will make the dress weaker (which isn't good for a playful 4 year old)

You can't really make something bigger because you don't have enough fabric to work with. You need to get the bigger dress and make it smaller.