When your witty back and forth banter screeches to a halt and your phone is as silent as a catacomb for days, do you automatically declare the relationship doomed? Don't. Give yourself a reality check! Texting is one method of communication, not the ultimate determiner of the state of your relationship. Until you know someone well, (IRL), you can't make an assessment of the state of your relationship based on text on your phone's screen. Do yourself a favour and quit the "he's just not that into me!" thinking (and while you're at it, donate your copy of that book to your local used bookstore if you've got one). Here's why he's probably not texting you.
He genuinely doesn't like texting
Some people genuinely think texting is a pain in the ass. Those people are right on a lot of levels. It's time consuming, easy to misinterpret, and often invites communication breakdowns. We're not all pro writers, and even fewer among us are masters of the 140 character form, (or however many characters your phone allows, you get the point.) We all have different relationships with their phones. Some people of us are glued to it, other forget it at home all day and don't think too much of it. You won't know until you know someone really well.
Long winded text banter isn't sustainable
The guy probably was into you when you had a whirlwind 4 day back and forth. He probably still is now, but texting at that rapid fire duration just isn't sustainable after a certain point. Keeping up a 5-witty-texts-per-hour momentum is pretty much a full time job. If your partner has one of those, texting may not be high on his radar between 9 – 5 PM. Being busy is a legit reason to not text someone back. Bear in mind that his thinking might more along the lines of "she should just know I like her at this point, text or no text." If you've established a real relationship, off the screen, it's reasonable to expect it will stay alive and well without constant texts.
After a certain hour, some people turn off "social me." If you're just starting dating someone, he's likely not going to write you back after having 3 pints at the bar with his friends, or if he's absorbed in Breaking Bad. Or doing that other thing we do at night – sleeping. It's not personal. Don't interpret it as such, until you have IRL evidence to believe something's up – unless of course your relationship exists only on your phone.