I think Adam Silver made a good point but it wasn't researched (I think). It came out of creativity. Shouldn't someone research why phone numbers are 7 numbers?
I'll tell you the answer via a hopefully, not-so-boring story and you, guys can consider what to do with the info.
When the telephone was first invented, phone numbers started with 4 numbers (this is important). Apparently, 4 numbers were the largest number easiest to memorize. But, as the population grew exponentially, they need to increase the number of numbers. Eventually, they made it 7 numbers because they ran a test and determined that most people could at most memorize 7 numbers at once. Due to this experiment, neuroscientists and psychologists concluded that the average brain had only 7 working memory "slots". In other words, we could only think of 7 things at once. Doesn't seem that much, no?
As time passed, they soon noticed something interesting happening. People, trying to memorize these phone numbers, had a tendency to arrange the telephone numbers into 3 or 4 groups. Why? Did our brains have some fascination with grouping? Scientists eventually figured out that our brain didn't have 7 working memory slots at all. Our brain actually only had on average 3-4 working memory slots and it was using a "trick" to increase the numbers we could memorize (eventually 10). People would unknowingly actually memorize the entire visual picture of 3 numbers into 1 working memory slot. Then, 3 numbers into another working memory slot. And, the final 4 into their last one.
So...if these zip codes are something that people need to memorize quickly at once, I would definitely not use a group of 5 and 4 numbers. The 5 initial numbers are too big for most people.
I would group them into groups of 4 numbers and make sure the numbers are less than 4. Something like that. You get the idea, right?