Um there is no Zen Vision that comes in 4 GB size, unless you're referring to the Zen V, which is a totally different player (Creative's answer to the nano)
Anways, I have owned both the Zen Vision M 30 GB and currently own the Zen Vision W 60GB. Both are great video players and solid audio players. My only gripe is that Creative needs to put more support for audio formats (i.e. firmware update) because there's no excuse for not supporting AAC or OGG when it supports such a wide range of video formats (Xvid, Divx, WMV, etc.). The Zen Vision series is superior to Ipod IMO because of the versatility of having native support for so many video formats. Also the battery life is better than advertised and it come with built in FM receiver and FM/voice recording. Sound quality is top-notch and the screen is bright and has more colors than the video Ipod. (over 200,000 compared to 67,000) The Zen Vision M is the better choice if you want the most compact player but the Zen Vision W is the better choice if you want a bigger, higher-resolution screen.