A single 4 tube fixture should do plenty well combined with the window. Naturally the closer to the lights the better, which is indeed tricky with various sized plants. The shorter plants/pots you could raise up a bit but it can become quite a balancing act lol! I think you will deffinately see a marked improvement in a few weeks time.
CRI is short for color rendition index. Basically the higher the number the truer the colors on an object compared to the object under sunlight. So theoretically a high CRI number would give you light that is similar to sunlight. The thing to remember though is that all these ratings such as CRI, Kelvin temperature rating etc are all based on what humans perceive. Plants see light in a completely different manner and only care about red and blue wavelengths the most. Blue is most important for growth so the K rating is probably the best to use to gauge how much blue light the bulb in question emits. The higher the K the more blue the bulb. But since red is also important to plant health you only want to go so far... Off the top of my head warm white is around 3k, cool white 4k sunlight/daylight 5k -6.5k
Are the sunlight/daylight worth the extra expense over coolwhite? Personally I think so but that is something you will have to decide for yourself if you want to shell out some more $$ for a bulb that will put out more light useable by your plants.