Hi earwig:
I'd definitely raise this with the teacher. 4c is NC Level 4 standard (government notional target for average pupil) - so she's well on track - but if there is no obvious problem, then slow progress is concerning.
How much reading does she do at home?
Does she only read one type (genre) of story or a range?
Does she read classic children's literature - which might stretch her?
Does she belong to a library/ get regular access to reading material?
Several things occur - but first and foremost - doing more reading is actually fairly straightforward the question is comprehension (which usually is included in the 'reading' category).
So it may be your DD reads fine but when it comes to it she's not able to recall key facts about what she has read (maybe forgets characters names, doesn't fully understand emotions/ subtext and/or may not fully follow the plot.
If this is the problem, then I'd seriously suggest trying to also be involved in reading at home yourself:
Have your DD read to you - but ask her if she understands more difficult words or what she thinks about a characters behaviour or decisions. As what she thinks will happen next or why would someone decide to do that? Get her thinking beyond just reading out words.
Consider looking into reading comprehension materials (the 11+ has lots of resources for this kind of thing available at most book shops) but the school may also be able to recommend something.
I have also realised that if you type in the name of the book, quite often there are free comprehension worksheets with books (e.g. Lion, Witch & Wardrobe - www.cslewis.org/resource/) - so always worth trying to see what you can find. Sometimes you can get worksheets - but without answers. This used to panic me at first, but then I just relaxed about whether we were getting 'the right answer' and decided it was more worthwhile to go through the process, right or wrong, to help DD1 with comprehension.
Finally many libraries have summer reading schemes which aren't just about reading books, but include worksheets/ book report forms/ etc... - it may be worth going along to your local library and see what they've got planned for this summer.
Links to great book lists:
The Book Trust
www.booktrust.org.uk/books-and-reading/children/
scroll down below pictures and select Book Finder - then select age & then you can explore books by genre.
Guardian Children's library - ages 8 - 11
www.guardian.co.uk/books/2000/mar/24/childrens.library
all sorts of great recommendations
Richard & Judy Children's Book Club
www.richardandjudy.co.uk/childrens-current-reads/Autumn-2012/229 - archive of book selection also through menus on bottom left.
Looks like it isn't being updated now - but still some great books there.
Guardian/ Observer - recommended reads ages 8 - 10:
www.guardian.co.uk/books/2011/oct/23/recommended-reads-children-8-10
& recommended reads ages 10 - 13: www.guardian.co.uk/books/2011/oct/23/recommended-reads-children-11-13
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My advice would be to keep reading simmering along over the summer - and really try to make a point 3-4 times a week to sit with your DD and discuss what she's reading, listen to her read & ensure that tricky words don't just get ignored/ glossed over - but that you really enjoy the process of discovering new words, their meaning & their origin.
HTH