In "The Blues Brothers", what is the significance of the food order?

There doesn't seem to be any special significance to the specific foods mentioned. The impression I got from the scene is that the "four whole fried chickens and a Coke/dry white toast" are what Jake and Elwood regularly order, allowing Matt "Guitar" Murphy to realize they're in the diner before he steps out of the kitchen and sees them. In other words, their unusual eating habits act as a humorous calling card by which their bandmates/friends can recognize them.

Other than "announcing" their arrival, the order might also be considered a reflection of Jake and Elwood's personalities: the four fried chickens and Coke match Jake's reckless, disruptive nature, and the dry white toast matches Elwood's generally rather mild and low-key nature.

SOUL FOOD CAFE

ARETHA Help you boys?

ELWOOD You got any white bread?

ARETHA Yes.

ELWOOD I'll have some toasted white bread please.

ARETHA You want butter or jam on that toast, honey?

ELWOOD No ma'am, dry.

JAKE You got any fried chicken?

ARETHA Best damn chicken in the state.

JAKE Bring me four fried chickens and a Coke.

ARETHA You want chicken wings or chicken legs?

JAKE Four fried chickens and a Coke.

ELWOOD And some dry white toast please.

ARETHA You all want anything to drink with that?

ELWOOD No ma'am.

JAKE A Coke.

ARETHA Be up in a minute.

[She goes back to the kitchen.]

ARETHA We got two honkies out there dressed like Hasidic diamond merchants.

MATT Say what?

ARETHA They look like they're from the CIA or something.

MATT What they want to eat?

ARETHA The tall one wants white bread, toast, dry, with nothing on it.

MATT Elwood!

ARETHA And the other one wants four whole fried chickens and a Coke.

MATT And Jake! Shit, the Blues Brothers!

[Matt goes out into the cafe.]

Referred to copy of script here for the relevant dialogue.