Is it proper grammar to refer to four digit number in hundreds?

This usage is informal but not incorrect. I wouldn't use it in official business documents, but it can be freely used in speech or less formal contexts.

While statements like "seventy-three hundred" are rarer than their equivalents for numbers below 2000, they're perfectly intelligible and frequently used.

I work in a building at '4400 North First Street'; I'd never say anything other than 'forty-four hundred' for the address.

It is perfectly grammatical, and often very sensible.

It is certainly proper grammar. From my experience, referring to four-digit numbers in the hundreds is more common in the US than in any other English-speaking country. As for 7300, seventy-three hundred is perfectly correct to say. It really all boils down to the speaker's choice. I must add, though, that for those numbers with three zeroes, thousand is preferred to hundred. Thus, 3000 is better called three thousand than thirty hundred!

In some places, city blocks are always referred to in terms of hundreds. For example, in Chicago it is normal to hear somebody talk about driving from the twelve hundred block to the seventy three hundred block of some street. This means going from a block with addresses of 1200-1299 to a block with addresses 7300-7399. It only works in cities with a regular grid layout where each street covers a range of exactly 100 addresses, but in that case the "hundred" is the basic unit, rather than the individual addresses themselves. So, you can sort of think of twelve hundred being grammatically analogous to twelve kilograms or twelve apples.

In general, I never encounter it in formal written documents. The only time I see it in writing is when somebody wants to explicitly capture a speaking style, such as an exact transcript, or dialog for a play. However, when reading a document out loud which says "7300," it can sometimes be idiomatic to pronounce it as seventy three hundred instead of seven thousand three hundred. ("Our Chicago office is located on the seventy three hundred block of Wabash street.")

I can't exactly answer but I can add that Steve Jobs is often heard talking about numbers in that way during his public keynotes. So I would say that it is proper business practice, at least orally.