Attorneys are of course the best-qualified people to advise you on this. If you are in Northern California, HN user George Grellas is the principal of a law firm that caters to a lot of startups:
http://www.grellas.com/With the proviso that I am not an attorney, here's a vary basic answer to your query:
a) If I read you right, you are the client and are hiring the design firm. I'm sure they're just trying to be helpful, but since it's your idea and your $, it should really be your NDA - you don't have to write it, but at least select it.
b) Don't assume that the plain English in the rest of the agreement is OK just because it's simple. 'Lawyer speak' isn't always necessary, but is wordy because it's precise and specific - and thus, predictable. 'Common sense' and 'obvious' statements usually depend on unstated assumptions, and those assumptions are not always well founded.
c) As far as it goes, this clause 4.1 actually looks good. It says that you own your ideas and that the design firm's access to them is limited as stated elsewhere in the NDA - which I assume specifies only what is necessary for them to perform their drawing and prototyping services. In short, this clause acknowledges and safeguards your rights.
Of course you want to keep costs down: most likely this NDA is fine, the design firm is ethical, and if you go to an attorney you'll feel like you spent several hundred or a few thousand on an umbrella when it's not even raining. But it's like the lock on your front door: it doesn't guarantee your security, it's often an annoying inconvenience, but if you don't have one you might as well just leave the door open.
The fact that you are wary of a clause which is explicitly written in your favor says to me that you are having trouble analyzing this objectively, which is only going to keep you awake at night. It's not a negative comment on you - most attorneys hire other attorneys for their personal affairs, because it's hard to be objective when it's your own interests at stake. your state bar can help with lists of lawyers, FAQs on engaging one for the first time, etc. Go with a small firm unless you are the sort of person who likes expensive department stores.