Yeah, it really depends on WHERE on 4th St you're walking from, and what your parameters for "walkable" are. I used to work near 4th St and University, and I walk fast, but here are a few places that I loved:
Cheese Steak Shop - mmmm, greasy, fattening, and fits perfectly into that "divey" category
Tomate Cafe - very very cute and charming, and the staff is totally sweet! I love the atmosphere--it's warm and sunny and homey
@Siam - ok fine, this is a stretch for walking from 4th, but I loooooooved this place for lunch. Clean, bright flavors, never falling into the trap of oiliness, and their lunch special is a great deal.
Cheese Steak Shop
1054 University Ave, Berkeley, CA 94710
2265 5th St, Berkeley, CA 94710
I enjoy Tacubaya for a quick breakfast (opens late at 10am) or lunch - it's nice to sit out on the patio - and you can stop in at the Pasta Shop to stock up on goods.
O Chame - cal/japanese fusion - I have been there twice for lunch and walked out happy.
Cafe Rouge is another option on the higher end - I haven't eaten there but pick up products at their meat market - here is the most recent thread - but it isn't super recent:
1782 Fourth St, Berkeley, CA 94710
1830 4th St, Berkeley, CA 94710
1788 4th St, Berkeley, CA 94710
re: Morton the Mousse
I second the Vik's recommendation. Just ate there last weekend and had the bhatura chole - huge puri w/ delicious chickpea curry. Various other chaats are good too. I think their dosas are pretty good, but I really don't care much for their uthapam - my friend had on this visit and I thought it was way too oily and had too cakey a texture - it reminded me of really bad, spongy pizza dough.
Overall though, Vik's remains a very impressive operation, definitely worth a visit if in this area.