We have a long description that is an extended/expanded version of the short description on the same product page ( first 3/4 lines / paragraph same content). Will we be penalised by google for duplicate content for this? will i need to rewrite both so there is different content in both?
Google is not aware how you're keeping your data, is it short or long description. It could penalize only for stealing content which you're going to use.
I guess that you're probably using short description with link "show more", which then displays long description. And that's okay, you should not be worried about that.
It is completely OK to display product descriptions the way you've described.
For duplicate content to occur there are must be at least 2 different URLs which fully or partially duplicate each other. In your case the content is found on a single page which cannot be considered as a duplicate description.
But you should still provide good user experience and don't show too much text multiple times on a page (first 3/4 lines is OK)