My Puppy of 4 1/2 months, has developed a worrying habit over the last week or so. He's always been good about his training, and obeying commands when we are at home. And the recall in the park was quite good.

However in the past week, he has taken to completely ignoring all commands on his walks. He will not sit at the kerb (which has been enforced since he was 8 weeks old), and when i let him off the lead he ignores me completely when i call him. Whats going on? I am now afraid to let him off the lead, in case i can't regain control of him.

He might not be ignoring your commands!  I think it very unlikely that he even fully understands them at his tender age

When you teach, for instance, sit you need to do it in a lot of different places and situations so that your pup understands this means putting his backside on the floor whereever he is, not in a particular place.  If you teach stays then you teach them at home, then go back to the beginning and teach in the garden, then go back to the beginning and teach in a carpark etc.

Have you really been taking him out for walks since he was 8 weeks old before his innoculations?

Remember he is a puppy, when you first take them out the centre of their whole lives is their owner and they don't want to lose touch with them.  As they get older and bolder they will explore.  Once they start to reach adolescence a around 10 months is when you really find out how good your training has been.

You can't train a dog in a few weeks, you can teach them but to get it ingrained can take months.

Do you think we need to go right back to the start with the training? even though he was obeying all commands previously on his walks? This is what concerns me the most- it doesn't seem to be a temporary lapse- i can't seem to get his attention at all on our walks, where as previously he always stayed close to me, and followed my voice and hand signals to him, he now doesn't even "hear me".

Training is an ongoing and continuous process for at least 18 months to 2 years old.  As has already been said no dog can be considered "trained" until it ALWAYS offers the appropriate behaviour on request, never offers the behaviour unrequested, never offers any other behaviour and the behaviour is generalised in ALL situations.  This will not be accomplished prior to 2! 

As a trainer you will pale into comparison next to the smells, sights and sounds of the big bad world outside unless you make huge efforts to be become a human slot machine :D

You don't mention what breed you have but some breeds go "deaf" quicker and for longer periods than others! :D

I agree with the last posters - I would just view it as normal part of the process - when you said he first hung on your every words he was only 8weeks plus, when concepts of independance dont exist in a young animals brain (virtually any species , i'd say), now at 4 1/2 months the maturing brain inevitably grasps this concept. I wouldn't worry too much, nor personalise it, just adapt and perservere. 
I'd defintley put him back on the lead and re-do recall training. Young pups have natural (whats called 'uncondtioned') recall due to dependance - older pups need to be taught 'conditoned recall'.
To set your mind at ease I can tell you that after 2 1/2 youngs of having a 3 1/2 yr old GSD permanenty off lead on the rescue centre farm I live and work at, I have just recently reverted back to lead training in certain situations, as her recall during 'the chase game' (farm cats and sheep) has weakened and delayed dramatically.

Hello,

I know what it does come now when i do write the words. You should train outside the house not inside. Why? The dog has to be distracted from all what is going around him. This does call teaching . In the house he does not need to go on heel or? What do you espect from a pup? heel sit work with 4 and a half month? The basic obedience does go thrue the intire life of the dog. The training should be done in 8 or 12 weeks, pup's do need longer. And you have to do it over and over and over again.

So go often out. go to puppy classes, plenty of distraction with other dog's and pup's and your dog does learn it. :o)