I am one of the many people who had mark 2 shuffles and was looking for an alternative product as I did not like the mark three, due to the earphone/controls. I use a shuffle all the time and need two as I like to keep different things on different iPods, one for music and the other for learning my foreign language. I tried a san disk but just could not get used to it at all after an ipod, it would not play all my music formats as I had imported in different ways using itunes, (no problem playing any of the formats on ipods), and I could not get rid of an embedded File "Oh Danny Boy", which kept blasting out as I was driving along listening to my Spanish phrases. I was soooooo relieved when the mark 4 appeared.
ADVANTAGES
The mark 4 has the advantage of course that it has the controls on the machine not the earphones, (vital if you are using it in a car as I often do). It has the advantage of the sound/speach feature, (tracks can identify themselves orally by pressing a button, allowing you to navigate to the track you want). It will auto sync, (which the mark 2 would not do). This takes a bit of getting used to if like me you have not seen this interface before, but it is worth getting used to, you can still add items individually, but it allows you to sync the last few episodes of your podcasts and all the contents of certain files, without having to do it manually. there is a live link between the iTunes folders, and your iPod and it automatically edits the contents of your iPod to include whatever you have asked it to keep up to date with. This is so useful, and of course you can add and delete things yourself manually in addition.
DISADVANAGES (only two very small ones, in my opinion)
I got so used to the mark two and I think it was a perfect size. I think that they have made this one a little too small. I find that as I pin the item on, I often turn it on because they have got rid of the blank space under the controls, and this was the part that your thumb conveniently pressed to pin the clip on. I find that it is harder to put on now, as you can not put your finger anywhere except on the control. I also do not like the fact that they have put the shuffle/play button on the same sliding switch as the on/off button because I tend to leave it rather than turn it off( because I invariably end up putting it on shuffle instead of play, as it is now the same switch, ( as I listen to podcasts and recordings I do not want shuffle as they must be listened to in order). I find that I am inadvertently switching it on, (due to not being able to touch the clip without touching the dial, and if I do not have my earphones on I am unable to tell it is on and I run the battery down. Perhaps over time I will get used to this but I have been spoilt by the perfect proportions of the mark two.
CONCLUSION - BUY ONE
Although there are some regressive design steps in my opinion, I would still recommend buying one as, there is nothing like a shuffle and the sync, and voice features are great additions. Perhaps apple will be able to tweak this design for the mark five, so you can clip it on without turning it on without your knowledge, - then it will be perfect!
UPDATE - PRODUCT LIFE - 18 MONTHS!
On 11 March 2012, my Ipod finally died. No problem I thought. Connect to the PC and reset it! The PC would not recognise it so I could not restore it. This means that it lasted 1 year and 5 months. It was used virtually every day though, sometimes for many hours. I have been using it to learn a language, and have been stopping and starting and rewinding a great deal and I do not know whether this has shortened its life or not. PERHAPS ANY OTHER READER WHO HAS A DEAD IPOD CAN ADD TO THIS DISCUSSION. I just thought that this information may be of use in my review. I am now using another Ipod shuffle. Although I would have expected it to last more than a year and a half, I am still a fan as it is designed perfectly for my needs.