Depends on overall design and load, and, in terms of screw/nails, it depends on what will be anchored to what. A floor plan annotated with fastener locations would help.
If you are notching in order to transfer load to the 4x4, a 1/2" deep notch is all that is necessary. Notching will take shear stresses off the associated screws/nails/bolts.
EDIT:
Great new drawing.
The main design considerations are the unit racking and collapsing (side to side), or the units tilting forward and falling over. Attaching the units to the back-wall in a couple places will stabilize the units, particularly their back portions. If the 1/2" OSB is screwed/nailed to the front and to the back 10' 2x6's, then the back-wall attachment will also stabilize the front portions of the units as well. If units can be attached to an end-wall, then attaching to both back and end walls will fully stabilize the units without needing to screw/nail the 1/2" OSB to the 2x6s. If back and/or end wall attachments are not feasible, I'd place 2x4 cleats on the ceiling and secure the 4x4 posts to those cleats.
On both the front and back, I'd put the 10' 2x6s on the inside of the posts, notching the posts 1/2" deep. This will give more support to the 1/2" OSB over the 24" dimension. Another benefit of internal attachment is that the 2x6s will be recessed 3" from the outside edge of the 1/2" OSB and 4x4s. Given that, you'll be able to hang all kinds of small things on the outside face of the front 2x6s without the items sticking out. You could also screw 4.5" wide lengths of 1/2 OSB (or anything similar, such as 1x6) to the underside edge of the 2x6s to create 3" deep shelves for light weight items.
If one end of the unit is open (such as facing the garage door, I'd consider moving the two 4x4 end-posts inward 16-30", which would cantilever the ends of the 10' 2x6s, creating 2-sided open access to the cantilevered portion of shelving.
The 5.5" of space immediately underneath the shelves will be kind-of wasted space. If one end of the unit is open, then I'd attach 24" wide OSB underneath the 2x6s to create deep pockets (cubby holes) open on that one end, for storing long stuff. The 24" long 2x6 end-caps can be under-mounted or top-mounted.
I found a matching image at http://www.furnitureinfashion.net/beech-home-wall-shelves-lasse-p-6628.html?osCsid=cf4c725a6d1c4b00d50adca495f827d3 which has dimensions of W85cm x D16cm x H47.5cm.
As to building it, I've built several like it over the years, and believe it really is up to you. What I mean is you can take this basic design and modify the dimensions to suit your taste. I use Excel and scale my plans until I'm satisfied before I even buy the wood. Also, you can build it out of any number of materials including: plywood, MDF, and solid wood. Also, the way you put it together is often dictated by your tools and skill. If you are a beginner and want to stain it, I would suggest you go with solid wood. It will be more expensive, but will simpler to build. I would also suggest you go to your local Lowes, HD, etc. and look at their selection of wood and see whether they have a hardwood you like. Looking at the picture, it looks like it veneered to resemble maple. But, I wouldn't recommend you use maple as it is too expensive. See if you can find another hardwood (I wouldn't use a softwood) that you like.
Another thing that will dictate your plan is what type of saw you have and your previous skill using it. Ideally you would want a table saw or radial arm saw for the cross cuts. If you don't have one of these, or have no prior furniture building experience, I'd recommend you either buy a pre-cut kit (if you can find one), or find a local woodworking business to cut them for you. Otherwise you'll likely end up with misaligned or uneven cuts that will detract from the appearance.
I don't want to sound negative, but, to me, even though this may look like a simple project, it's not a beginner project unless you can be satisfied with less than perfect work. If you do decide to go ahead though, just don't expect it to be perfect or you may be disappointed. Just enjoy the journey and hope you get to where you want. Also, there are tons of web sites that provide free info on cutting and assembling bookshelves. Although they won't look like this, their planning, cutting, assembly and finishing techniques will give you the knowledge you need for the project.
Best Answer
I've always wanted to do something with a shelf attached with cables to pulley blocks with counterweights. Kind of old school, but would do the job for you. Using counterweights is good in your situation, because the weight of the empty appliance (microwave with no food) is constant. The downside is you must spent a good deal of time designing the system to the fit and finish of your kitchen. It looks like you can get the cable and pulley blocks from amazonsupply.com.