My 4 year old got this as a Christmas present and loves playing with it. It has larger pieces than standard Meccano which are also flexible and is therefore easier to build, but still quite challenging.
At first my son needed quite a lot of help but now, having built everything a couple of times he can do this mostly on his own, with occasional help needed tightening screws. He's learned to follow the picture diagrams and use the screwdriver and spanner, but I'd say the age suitability would depend on the child. My son's quite good with his hands and tends to enjoy fiddly little things (like Hama beads). I've noticed on the Meccano website they have now changed the packaging to say from age 5, not 4 as the amazon picture shows, but I would recommend it for a 4 year old, particularly if you have time to help him/her with it the first few times it's used (and you might even enjoy reliving your own childhood memories of Meccano!)
THis kit can be built into 4 different designs - all are lovely, sturdy vehicles which a child will enjoy playing with for a while before re-building.
Definitely a good buy as it has been played with over and over again since Christmas.