A Single Sentence Puzzle

One of my peculiar acquaintances introduced me to the world of puzzles, so I've started to dabble in making some of my own.

Here's a single sentence puzzle, where I want you to find a four letter word that is the answer to the puzzle, and a four letter word that is NOT the answer to the puzzle. You will be very sure when you have found either word.

I_ Y__ __AD TH_S RI___, _H_ A_SW_R ISN'T ___N THE A_SWER.

Oh, by the way... there's an L missing somewhere.

Hint for the Not Answer:

The numbers have two uses. One interpretation of the numbers will get you the answer. Another interpretation will give you the not answer!

Answer

The missing letters, filled in, give

IF YOU READ THIS RIGHT, THE ANSWER ISN'T EVEN THE ANSWER.

The letters added were

FOUREIGHTTENEEVEN

The missing L noted in the puzzle comes in here -

FOUR EIGHT TEN ELEVEN

The answer

is OATH as found by @Deusovi -
it's the 4^th, 8^th, 10^th, and 11^th letters of the sentence.

The NOT answer

is THIS as found by @Sherlock9,
because the 4^th, 8^th, 10^th, and 11^th words of the sentence tell us THIS ISN'T THE ANSWER.

If you upvote this answer, please make sure to upvote Deusovi and Sherlock9 as well!