Impedance Matching - Tube Amp/Speakers

Hi, I am not technical guy, so excuse my questions. This weekend my friend did check bias on my VTL ST-85, which I recently did buy, and he also measured my speakers Dunlavy SM-1. Multimeter showed him that they are 4 ohm spekares (althought, on paper, they should be 6 ohm). He told me that this is not good combination - 4 ohm speakers and tube amp. So, is it safe using VTL ST-85, or tube amps generally, with 4 ohms or 6 ohms speakers? I am really confused because I like sound, but also dont want damage my amp or speakers.