Ok so I am also planning a 4 month trip to europe and just wanted to let you know some things I read… theres a thread on here somewhere that gave great tips for girls backpacking and one thing she said is before you go every once and awhile go to your local mall and go to the makeup people explain your situation and ask for free samples. Also.. Ive read alot of things that say shampoo and most things like that will be easy to get over there.. so just bring one bottle of shampoo and one of conditoner .. they may not have your brand over there but they do have alot over there.. same goes with TAMPONS! haha.. About the clothes things my friend went for 2 months and she said she was good with about a weeks worth. She bought alot of just basic layering things in different colors, like tanks, thing long sleeve or 3/4 sleeves, etc. And just like 3 or 4 pairs of bottoms. Jeans, khakis, etc. She also gave me a good tip because I live in florida and im a big flip flop girl dont freak about buying tennis shoes… where what your comfortable in def bring walking shoes for days where youll be doing alot more walking or hiking or what not.. but youre going to be alot more comfy in whatever shoes youre use to wearing. 

you should read this post here
 
http://www.eurotrip.com/forum/tm.aspx?m=14413
 
It should answer most of your questions

I wouldn’t pack too many more clothes than what has already been suggested. You’ll just have to do laundry fairly often.  Maybe consider a different hairstyle? I wouldn’t want the hassle of long, knotted hair on such a trip.

I’m planning a four month trip this summer too.  I live about 30 minutes from the AT (Appalachian Trail), and after backpacking trips galore on that thing, I have no doubt that a week’s worth of clothes will suffice.  I’m not so much bashful about having some extra spots of dirt on my pants; socks are another story.  I’ll probably pack two a day.  Nothing can ruin a trip faster than wet/raw/uncomfortable feet.  Also considering I’ll be doing LOTS of walking this summer (I’m bringing a tent and bag, and plan on camping a bunch), I’ll have to keep in mind wear and tear on socks.  I harp on those, but they’re important!