Flash from power tube

Brand new to tubes here. Just recently bought a Jolida 302BRC here on audiogon. Previous owner put in Winged C Sed el34 tubes. Been working great since feb, but today when I turned it on, one of the tubes flashed and the amp wouldn't turn on. Stupidly I tried again, and same result. I removed the tube but don't really know what visible evidence there would be since i don't know what to look for lol. Anyway, my question is can I just order a replacement tube? Or when replacing tubes am I supposed to replace all 4? I know they were purchased at the tubestore.com so I could easily order 1.