Space joints (in feet) no more than 2-3 times the slab thickness (in inches). A 4" slab should have joints 8-12 feet apart. Idon't like to use hand tools, because you usally don't get a deep enough joints with the jointers you see at Home Depot, Lowes, and such. For a 4" slab you'd have to utilize a commercial grade jointer, and for the price you'd be better off renting a concrete saw. There are 2 types available. one that is hand held ( looks something simular to a chainsaw ) and one which is on wheels that is usually more precise than the hand operated ones. Depending how many joints and the thickness thereof, you may want extra blades. You can save money on the hand held, if your confident, go for it. Either way, cut joints deep enough to be 25% of the depth/thickness of the slab.
Make sure to utilize expansion material, such as asphalt board to cut your own ,or other commercially available, (already 4" height) also there are quality materials sold online in rolls, or available through the concrete ready mix companies. for this purpose. Use anytime between an existing strcture and your new slab, or between an existing slad and your new slab. Or it will cause problems like cracking or even leaks if your going up against a foundation, because the old and new will expand at different times.
As far as the mesh goes, either type is exceptable, a lot depends on your slab size, because the rolls are sold in 150' lengths and there may a lot left over. The mesh should be placed in the middle of your pour (bottom to top)so you'll avoid any rusting, which happens when wire is placed too close to the top of your slab, or in the event that you get cracks later on down the road. there are a few different ways to accomplish this. You can pour half of your thickness first, and then put your mesh down, followed by the rest of your concrete. A second method involves putting your mesh down and using a hook of some sort to pull the mesh up halfway as you are pouring the slab. There is a third method where commercially avaible mesh and rebar chairs are used to set the rebar and or mesh on. This process isn't used much though for just a residential slab.
Anouther thing to consider is, if you want to utilize plastic and pea gravle as a part of your project. The pea gravel is used for drainage, and a good subase for support. While a good 6 mil plastic is used as a vapor barrier. Either way, you should compact the subgrade( on good dirt) or the subbase if you use pea gravel. The subase isn't essential if you have good clean compacted soil. However it's a good idea to use 6 mill plastic as a vapor barrier which will help if your in an area where tempatures dip below freezing. Good Luck!