It says Input 18 watts; Impedance 4 Ohms. Which amplifier should I buy?

It depends on what you are talking about. I would assume you are talking about a loudspeaker, but it also depends on whether you are using it for a guitar and whether you want tubes or transistors, whether this is a bass, midrange, tweeter, or wide range, and where you intend to use it.

If this is a general purpose device for home or car use, I recommend the amp I use:
smile.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=Lepy+LP-2024A