The given terms can be divided into two sections. First section is for the numbers and the second one for the letters.
1st section: 1, 2, 3, and 4
2nd section: I, L, F
I is the 9th letter in the alphabet, L is the 12th letter, and F is the 6th letter in the alphabet.
The pattern observed:
I to L: 12 - 9 = 3
L to F: 6 - 12 = -6
Next letter should be +9. This means that the letter should be 9 places higher than F. The letter should be O which is the 15th letter.