how light or heavy bullet can i use in a 14.5 m4 barrel with a 1/9 twist ?

as heavy as you want, test fire several types and weights of ammo to see what you gun does and doesn't like. generally 1:9 twist like the lighter bullets and the 1:7 twist will take heavier ones, but again all guns are different.  
Is a 3 in group acceptable to you at 100 yards if not then try a lighter bullet.
I recommend starting off with some 55 grn and working up from there. 62 grn may be all your gun will stabilize of maybe you can go up to a 69/70 grn bullet. who knows till you try it at the range.

Most any 1/9 twist barrel should have no trouble stabilizing bullets up to about 73 grains.  Some particularly good ones will handle bullets as heavy 75 grains.

Granted, each barrel will have its "favorite" loads.  So, if maximum accuracy is your goal, you'll need to experiment to find a balance between accuracy and suitability for your purposes.  But, generally speaking, you should be fine with any of the "standard" loads. (M193, M855, or any of the commercial loads 69 grains or lighter.)

It isn't the weight that matters. It is the length. The longer the bullet, the tighter the twist must be to stabilize it.

The 1-9 is the in between and should stabilze anything but the 80-90gr match stuff at long range. With your shorter barrel I would try to find something in the 62 gr weight it likes and that should be the best of both worlds. your shorter barrel will likely have issues with real heavy bullets especialy at longer (300+ yards) ranges. My Armalite SPR 1-9 really likes the federal 62gr Fusion and Black hills 69gr OTM Sierra Matchkings but the barrel length is 20". It even keeps pretty decent groups with the 75gr OTM at the 400 yard mark.