Hello Katalax:
First off I would refer you to the MN pages on progress through NC Levels here: www.mumsnet.com/learning/assessment/progress-through-national-curriculum-levels
3b is the notional end of year target for end Y4 - so she's doing well in maths/ reading but possibly struggling a bit with writing (3c instead of 3b - but odd that she's moved back a level and that is worth querying with the teacher).
So I think my main advice would be to relax - she's a bit behind in writing, but doing o.k. otherwise....
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In terms of writing, it could be that she is not writing enough in her work or that her punctuation/ vocabulary (openers, adjectives, range of words used) is more limited than it should be. I'd talk to the teacher before the school year finishes and ask if she has ideas on what you could work on over the summer.
I know with DD1 we had real problems moving from what was basically a monosyllabic description of things to a more interesting read for the reader.
I found these writing (VCOP) pyramids really helpful: displays.tpet.co.uk/?resource=387#/ViewResource/id387 - basically the top of each side of the pyramid is the lowest ability in writing and the bottom of each side of the pyramid is the highest ability. These simple Vocabulary/ Connectives/ Openers/ Punctuation pyramids really help you to visualise where your child is at and what they might need to incorporate into their writing next.
I found two things useful at this stage:
We played 'CAN WE MAKE THAT SENTENCE MORE INTERESTING' -
I did this with homeworks and just now and then (often whilst waiting on DD2 to finish dance lesson). So for example DD1 wrote:
Harry Potter is a boy.
(I yawned and said come on you can do better).
DD1 wrote: Harry Potter is a boy and a wizard.
(Better but I'm still bored)
DD1 wrote Harry Potters seems an ordinary boy, but is a wizard.
(Oooooo I said. That's a bit better).
She then had a think and came up with:
Harry Potter may seem an ordinary little boy; however, he secretly is an amazing wizard.
(I cheered! That's more like it!)
Getting them to see that what they're writing needs a rhythm - can't be all long sentences or all short sentences, but needs a bit of a mixture and perhaps a few more descriptive words is a real struggle - but keep working away at it.
We did find the CGP workbooks really helped on many levels: obviously grammar/ punctuation but there was a lot in there on spelling rules/ spelling changes with different tenses. here's a link to the Y4 workbook on Amazon (there is also a Y5 workbook): www.amazon.co.uk/KS2-English-Literacy-Workbook-Year/dp/184146158X/ref=sr_1_1?ie=UTF8&qid=1403268868&sr=8-1&keywords=CGP+literacy+workbooks+year+4
In terms of getting your DD to write more:
Diary (buy a nice notebook & pen and encourage her to write in a diary)
Postcards: when you go places have her send cards
Thank you cards (for birthdays/ treats or outings with friends/ relatives)
Fan letters: Have her write to her favorite author/ tv show
If the school asks for a summery scrap book projects - insist that she writes a bit more than usual, don't let her get away with only a sentence when she could write 3.
HTH