If we are just talking snacks (on the go type stuff) then maybe dehydrated fruits or even jerky (talk about concentrated protein!!! )

I feel your pain! I really do.. my now 7 year old was a very picky eater at 4. Even now she is a very plain eater, no sauces or strong flavors, but she eats just about everything.

Others have been giving you great suggestions about increasing protein, but I'll tell you what I eventually had to do... I got tired of trying to sneak in protein, and just stopped catering to her tastes. After I determined that it was not a true sensitivity issue, for example if she just hated the texture of strawberries, I just decided it had to change. Our meals were revolving around her... also if you do require her to eat a certain amount, say it once and then don't continue to focus on her for the meal. Make a big deal out of her eating well, but take the dynamic of her pickiness out of the center of your family meal.

I had to go old school, she had to eat at least half of what everyone else ate or she lost TV or a playdate. I don't make her eat liver or brie but she can eat the same beans or ham that everyone else is eating.

She also had to try new foods.. For example she always refused to eat peanut butter, she tried almond butter and loved it and now eats peanut butter.

She has tried blackberries and raspberries a few times and really does not like them so she doesn't have to eat them, she's not crazy about chocolate either or sauces, but she now eats pretty much every meat, fruit, grain and veg we eat.

At a dinner party recently a couple told me that they order extra pizza sunday night because the only thing that their 9 year old son will eat for lschool unch is a slice of cold pizza, so all week they fish out a piece of cold pizza out of a cardboard box. He also has a limited menu he will eat in addition to pizza. You don't want to end up in this situation, if it is not extreme dislike you need to get her to eat regular foods. Real life gets in the way and eventually you won't have the time to be soaking millet and grinding up cottage cheese for her eggs while you are getting ready for work. The kids I know that are older and who are allowed to be picky just end of living off of granola bars for breakfast and snacks, which are not that great of a substitute for real food.

I do realize that she is young and that you need to get her to gain weight, so a two pronged approach is probably best, where you are supplementing and doing SMALL behavior modifications to get her to eat.

Try pancakes made with 2T flax meal and a scoop of vanilla or chocolate whey protein powder mixed in -- you can see the flax meal, but there's no texture or taste to it, and the protein powder flavor is not overwhelming at all (it adds a sort of sweet malty flavor).

Also, google around for mini muffin recipes with fruit/veg in them -- I do zucchini muffins, and also these crazy green muffins that incorporate a spinach/strawberry/banana smoothie puree (DS thinks they have green food coloring in them). The key is to puree the fruit and/or veggies until they are completely smooth. You can also add flax meal to the muffins for extra omega 3's and fiber. Put 3 chocolate chips atop each mini muffin, to help distract and encourage consumption!

If she'll eat scrambled eggs, mix in some small curd cottage cheese (you could also hit the cottage cheese with an immersion blender to really grind it up). No taste, no real difference in texture, but you're adding protein and vitamin D. You can also add cottage cheese to pancake mix, in lieu of some of the liquid, but it does change the texture/taste a little bit so you'd want to play around (for me, the immersion blender plus some vanilla extract works great).

Finally, will she eat red sauce on her pasta? If so, again google around for recipes. You can get a lot of different veggies -- and thus a lot of fiber/nutrients -- into red sauce without really changing its taste or texture at all. I've made some with butternut squash, zucchini, and spinach all ground up with the tomatoes and spices. Again, the immersion blender is your friend here.

Chobani makes greek yogurt in kids flavors/cups and also yogurt tubes. We freeze the yogurt tubes and they are more like an ice pop and my DD likes those a lot. The Target by us carries them and occasionally Giant.

Have you tried savory lentil pancakes--good source of protein and lots of Indian cooking websites have recipes. You can just cut down on spices to make it more kid friendly.

Oatmeal is also filling and a healthy snack topped with blueberries or any fruit she will eat.

And, I second the idea of offering foods she has rejected before. My 3 year old rejected string cheese for almost a year and now he loves it. We kept offering it to him bc we were also desperate for healthy protein snacks.

First, do some research online to see how much protein a child of her age/size should be eating. I did this for my picky eater (8.5y) and it was really helpful. The CDC and Baylor College of Medicine both have info online and are reliable sources.

Second, anything that she will eat, read labels VERY carefully to find the mot nutrient dense option. Example: Trader Joe's WW Cinnamon Raisin English Muffins have 6 grams of protein in them. That with butter and one turkey sausage link is my DD's breakfast every day. Arnold Protein bread has 7g protein per slice. They're big slices and you can easily cut one in half and make a small sandwich that packs a decent nutritional punch.

If she will eat yogurt, make sure you are getting Greek yogurt - the real stuff - it has 3x the protein of regular yogurt.

Cheese

Slather butter on anything she'll eat with butter, pasta, toast, her waffles, etc.

Avocado - try cubed, try mild guac, google the chocolate pudding recipe that uses avocado

I have a GF cake recipe that is basically eggs, chick peas, chocolate, and sugar. It's really good - even DH did not guess what was in it. Maybe reduce the sugar in it and make it as muffins instead? http://allrecipes.com/recipe/garbanzo-bean-chocolate-cake-gluten-free/

Good luck. It's really hard, I know.

Since you are feeding her pasta, I would try the Barilla protein plus stuff. It tastes very similar to regular pasta IMO -- we prefer it.

Good oatmeal has decent amounts of protein. We get the Bobs gluten free and I think it has 7 grams of protein