Yes, you should absolutely use all or at least most of your witness votes (4 empty slots is not a big deal) for the simple reason that it makes the chain more secure. By default, vote for any witness you believe to be non-malicious.

The smaller the number of total votes in use, the easier it is for an attacker (for example, someone who hacks an exchange and steals their STEEM) to vote in their own witnesses and cause havoc.

Of course, using all your votes does not preclude changing your votes in favor of a witness who, as you say, brings something to the table.

The safety and security of the chain (which means your own investment) benefits from as many people as possible using all or at least most your witness votes. In that sense, any responsible witness brings something to the table.