Re: Do I need to download sims4 base game both on origin and steam?

@Cecile-Adele  If you want to play Sims 4 through Steam, or use any packs bought through Steam, then yes, you need to install the base game specifically through Steam.  You don't need to install it through Origin though: Origin will recognize the Steam install and act accordingly, even allowing you to install your Origin-bought packs.  So it's best if you uninstall the Origin copy of the base game and then install through Steam.

If you already have user content, e.g. saves, you want to keep, move the Sims 4 folder out of Documents\Electronic Arts before uninstalling.  This folder shouldn't be deleted, but it does happen sometimes.  After the game is up and running again, you can copy your saves and any other content you want to use to the new Sims 4 folder that will have been created.