Willeseden;
Check for Dupuytren's contracture as a cause of clawing of D4 & D5; this may be unrelated to the muscle wasting.
If the thenar muscles are not atrophied & weak, then it is unlikely a problem at the neck (e.g., a C8 or T1 root lesion). Is the flexor carpi ulnaris weak? How about the flexor digitorum profundus to D4 & D5? If they are both ok, and the weakness is limited to the hand alone, then suspect an ulnar nerve injury at the wrist. If these are also weak, then suspect an ulnar nerve injury at the elbow.
Is the other hand ok? If there is any atrophy in the other hand, look at the tongue; check for weakness and fasciculations. If these are ok, then it is not likely ALS.
Is sensation really intact in the ulnar nerve distribution? I've had several patients with severe ulnar nerve compression, but preserved 2-point discrimination and light touch. I can't really explain it, but sometimes the motor fibers are much more involved than sensory.
Other than checking these issues, an EMG & NCV would really help clarify.