Well a lot of people including everyone currently using a B4 would think the B4.1 id just a kit with a few new bits added in.
Well it isn't. So so many new parts and so many changes.
This is a small list of the changes and I will try and post some pics laters
Chassis - for sure of different material and more flex and additional strength up the front nose (although I have never had a problem there)
Comes with the all new V2 shocks and they build like a dream and very very smooth
Comes with the new slipper clutch and a great crystal clear glass type plastic gear cover - absolutely awesome - love this!
New front and rear shock towers although they have been out for a while
Comes with 30deg caster blocks that have 3 holes instead of 2 for adjustment
The ball studs are all new and adjustable from the top with allen driver
Now comes with blue thumb screws included in the kit
Carbon fibre battery brace that i think was included in the previous factory team but this also comes with split foams for fine battery adjustment front and rear
New squat rear plate
New style rear wishbones
New rear drive shafts like the B44 and locking ring for the pin to stay safe
New rear towing blocks outer and inner. The rear end is now FULLY adjustable for tow in and also a great combination of outer and inner tow in can be achieved
Included is the Proline body and 2 rear wings (taste on this one but look real nice)
Anyways, I am sure other items that I have forgotten but i have built the kit and has gone together like a dream. Even to the point of I didn't get the files out to make those little free up jobs. Just perfect and about time a manufacturer has made a kit that goes together easilly and freely and for sure doesn't make one feel that they need to buy anything.
Finally, I added up the differences and the value of them.
It would cost well over £220 to buy the changes so just buy the kit - Simples!!! 
I know loads are being sold as we speak but I think Marc has some still www.m-k-racing.com
Please check with him though
I will post some pics later