Not a fan of sandwich, unless you are going to go with a 6x6 post and notch the post so the beams are sitting in the notches. If you just sandwich, the support of your posts rely on the bolts attachments. Check with your local code to see what is required.

We used a notched wood post beam connection, but you can't do that with a 4x4 post. How far off the ground is your deck going to be?

Here is a link that might be useful: Notched Wood Post Beam Connection

Current code says:
1. Use 6x6 posts, not 4x4.
2. Don't sandwich beams, either set them on top of the post with a post beam connector, or notch out the posts and set them on top of the horizontal part of the notch.

Lots of people made decks in the past (and many probably still do) with sandwiching, but post caps are better.