Each person's hand/finger configuration is different. Therefore stretch/reach problems can be due to short fingers/small hands and/or problems bending the wrist. If the wrist bend is the only problem, then the bent neck should solve it. If stretch is the major problem, then an added key or keys will help. Hole 4 or holes 3 & 4 are the usual suspects for stretch problems.
You can check to see what you need by holding a tenor up with the mouthpiece by your right ear. This is about how high the instrument would be with a bent neck. It puts your hands in the position they would be in when playing an alto. Now see if you can reach the holes/keys. If so, the bent neck should solve your problem.
If not, then check to see if added keys will help. If the right hand stretch is a problem, then, with the mouthpiece in your mouth, put your 1st finger about 1/4 inch away from the middle finger and see if you can reach holes 5 and 6 and the keys on hole 7. If this works, you only need a key added on hole 4 and no bent neck. You can confirm this by taping over hole 4 and seeing if you can get the lower notes to sound clearly. If it doesn't work, see if it works with the mouthpiece by your right ear. If so, then you need a hole 4 key and a bent neck.
If you have left hand reach problems, then try putting finger 3 about 1/4 inch from your middle finger. If you can now cover holes 1 & 2, then a hole 3 key will solve your problem. You can confirm this by taping over hole 3 and seeing if you can get the lower notes to sound clearly. A bent neck is not necessary for hole 3 stretch problems.