Some take longer than others to get going. I have a couple that are only about 7 inches tall, right beside others that are about 2.5 feet tall...and they are all the 4 foot varieties.

If you want bushy plants with lots of blooms, leave them as they are. If you are growing for show, and want fewer but taller and bigger, then you can select the best stem, and cut the others off. It is your call. Personally, I like bushy plants with lots of flowers, so I leave them.