I've been meaning to post about this for a while now and never got around to it - matching ORT book stages to book bands is somewhat more complicated than most parents realise (and schools - even reading chest just matches the stage to the book band colour).
ORT created a PDF (in 2011 according to the copy I have on my computer) which matched their individual titles to book bands. Typically they've taken it down, however I've managed to link to a cached version, which although not a pretty, still contains the relevant info.
This shows that stage 4 Fireflies actually go from blue to purple and stage 5 from green to gold. Of course your DS's teacher may or may not realise this (DDs previous school didn't, current school seems to have their scheme following the information contained in the link below). As Fireflies are nonfiction could she have chosen the books due to the topics they cover matching with topics in other lessons (imagine a hopeful emoticon)?
Newer versions of these books will have the book band colour as well as the stage, but I don't think the older versions do - certainly there seems to be some confusion as people usually equate, for example stage 9 with gold, stage 10 with white and stage 11 with lime, which is roughly how it goes, however some stage 10 books are gold, stage 11 runs from gold to lime, 12 white to lime and only at stage 13 are all the KS1 books lime and this variation is more marked in the middle stages of the scheme.
If I were you I'd be inclined to ask her firstly whether he has ever failed to answer any of her comprehension questions correctly and if so what sort of questions were they - if you are not comfortable with this line of questioning then get some detail about exactly what it is she wants him to be able to do to demonstrate this improved comprehension she is seeking from him. The result, hopefully, will be the same, either she'll provide you with the detail because there are areas he needs to work on, or she'll struggle because it's more about holding him back while the others catch up .... or possibly some other reason I haven't thought of yet 
Here's the link.
PS sorry for epic post length!