Would an old iPhone 4s with a SIM card be sufficent for phone calls only when abroad?

Servant of Cats is correct. In most parts of the world, an iPhone 4s won't be able to access cellular networks, so not a good option to travel with.

Also, you posted your email address at the bottom of your post. Fortunately, the Moderators edited it (hopefully enough) to mask your identity. This is a user to user forum, visited by millions of people from all around the world. It's NEVER wise to post personal information, such as your email address on a public forum.

Probably not. That phone only supports 2G and 3G cellular networks, which in many parts of the world, carriers have shut off.