Mumsnet has a basic sheet about progress through NC Levels here: www.mumsnet.com/learning/assessment/progress-through-national-curriculum-levels
This is for dibbletribble who asked about her DC's scores:
Basically, as it plays out here (and as MN describes) a school generally expects 1-2 NC sub-levels progress per year in KS2 (see other feed on whether progress is 2 or 3 NC sub-levels per year here www.mumsnet.com/Talk/primary/1708676-Is-it-3-National-curriculum-sub-levels-or-2-per-year.
So at start of Y4 4a (let's assume achived) - should finish 4b/ 4c
Y5 (at minimum starting 4b - so should achieve 5c, maybe 5b)
Y6 (at minimum starting 5c - should achieve 5b maybe 5a).
The writing score is slightly lower (3a) so that's borderline for achieving a NC Level 5 - not impossible but safer to predict high 4 at end KS2.
My gut instinct is where your DC is a 4a - that they should be a secure 5 by end of KS2. That means your child is performing above average. Obviously in a given year the number of pupils performing at NC Level 5 will vary. Only a very small percentage of pupils will be a secure Level 6.
It entirely depends on the quality of Y5/ Y6 teaching, support at home and work from your child whether your DC will achieve a NC Level 6 in English or Maths. It is possible but it just depends on so much. What I can say is there is strong likelihood of at least a NC Level 5 (which should feed through to a good start at senior school).
My understanding in this LEA (so may be different where you are) is children have to show attainment at L6 before they will be elligible for NC Level 6 SATs (info from DfE on L6 Sats here: education.gov.uk/schools/teachingandlearning/assessment/keystage2/b00208296/ks2-2013).
Perhaps teachers can confirm but my understanding is that L6 results were previously not reported but will be now. In general I'm under the impression there is a push to get pupils to NC Level 5 achievement now, rather than 4 (see recent discussions on 'secondary ready' - e.g. www.education.gov.uk/inthenews/inthenews/a00222410/new-primary-school-measures-to-raise-ambition-and-standards
HTH