If you're doing monopitch, 10 degrees is favourite- enough fall to drain but still classed as a flat roof & means you're less likely to fall foul of height restrictions.
Rafter spacing- 600mm usually. My joist software is coming up with 47 x 225 which seems complete overkill to me, others may comment. That does include capacity for you standing on it for maintenance as well as snow load- personally I'd use treated 47 x 100, you'll be standing on the concrete when you build the thing and maintenance ain't going to happen is it, plus the concrete roof is doing the real structural job.
Your cheapest way to do the monopitch is probably to mortar 4 runs of concrete blocks along the 'high' edge (and do some returns down the eaves) and a run of bricks along the low edge then fix wall plates to them- on a 4m span you need 700mm height difference for 10 degree pitch. You MUST strap the wall plates solidly to the garage structure- wind uplift is a scary and powerful thing. You can make a timber frame if you want but it'll probably be more expensive with all the strutting you'll need plus the bits of timber sitting on the concrete roof may end up sitting in water which is not a good thiing.
Then birdmouth and skew nail your rafters to the wall plates
Then membrane over the rafters- if you're going to batten IMMEDIATELY then staples will do to hold the membrane in place but 12mm clout nails are much better at resisting tearing if the membrane is going to be flapping in the breeze for a day (week) or so
Felt support trays go on the low ends of the rafters under the membrane- they support the membrane where it falls into the gutter and keeps the job watertight when the membrane rots after prolonged UV exposure
Then battens- 25 x 50 are my favourites, for tin sheet or fibreglass 500 centres at most, for artificial or real slate follow manufacturers' spec (though most slates won't work well at that pitch without slate hooks, which are annoyingly expensive)
Then your roofing sheets
Then side and ridge trims and fascia boards
Then guttering (your existing gutter will be too low)
Job done. Not going to be a cheap job either- you need about 78 metres of 47 x 100, 80 metres ish of 25 x 50, 35 sq m of roofing sheet (you have to overlap the sides, when you buy your sheets check what you're getting- 1 metre width is NOT the same as 1 metre cover), 50 sq m of membrane, 17m ridge/side trim, 16m fascia board .........
There are other solutions- you could just get a load of bitumen and a tar boiler and coat the concrete roof with a half inch deep bed of the stuff.
