Each of the words an, am, at, come, rate and many others can be transformed into a new valid word by adding a prefix be.
The following list consists of 16 words, which may be sorted into 4 groups, each of 4 words fitting the same two-letter prefix, specific for that group.
One such group needs KN as a prefix.
Other 2-letter prefixes are up to you to find.
All 16 words must be used.

1. able
2. aid
3. air
4. and
5. ant
6. art
7. ash
8. ave
9. easy
10. eel
11. idle
12. irk
13. other
14. out
15. own
16. ows

(In reply to re: first thoughts by Dej Mar)

 

Charlie, I wanted to write you a short note about the validity of the word KNOUT , but after reading DM's post , I have very little to add. The word is of Russian origin, means "whip"  and is in common use, pronounced knoot (non-silent k, like in kite). It is accepted by many dictionaries , inter alia Britannica, Oxford, M-W et al.         
Btw, I master Russian on the level of my English and can  testify that there is no synonym describing whip (in English there are several- look it up!  

While creating  a word puzzle I always try to introduce some new words, not necessarily esoteric but not-so-commonly used.  Quirk and knout  in our case are such words, adding some educational value to the process of solving.

 

DM, re: knout – I agree with your remark.

Actually, this is the only thing that I agree.

 

In regard of multiplicity of possible classifications:
a.  Nowhere I imply the existence of one unique solution;
The solver is requested to provide a 4x4 classification, fulfilling   certain demands (see Charlie's solution, mentioning 2 variants, - might be more!). There is no warning like "provide the solution and prove there are no others", which is sometimes a basic demand in math problems and totally redundant and   even impossible in word problems. If I were to place a disclaimer regarding quantity of qualifying solutions it would be: "Since the riddle was created bottoms up - there might be answers beside the original one . If you find them ,OK -
THE MORE THE MERRIER

Charlie's programs provided unexpected solutions  on several occasions .

b. You claim that there probably 4 possible solutions,
 I believe there exist much more, but you provided at most one correct answer:  in the  first
  BRAVE & KNAVE create a contradiction
  I did not bother to check the second, leaving this task to you.

c. Two pieces of advice: 
*Aim your skills (which I hold in high esteem) towards accuracy in solving rather than nitpicking i.e.  trying to find faults, real or imaginary.

*Use the rating system by the relevant criteria: creativity, innovation, interest, is it challenging or not, does it broaden solver's knowledge   and if you do it you will find that "4 groups of 4" puzzle deserves mark much  higher than 2.

If you have other parameters of evaluation, I would like to know them.

 

Posted by Ady TZIDON on 2016-08-17 12:28:47