Re: does this lamb shift only exist at these levls?

The Lamb shift is due to  "radiative corrections" (mainly the emission and 
reabsorption of a virtual photon) that remove the degeneracy of the 
np(1/2) and ns(1/2) states in hydrogen by shifting the energy of all s 
states.  This shift does occur for all n=2,3,4,...   However, even though 
there is a Lamb shift for all n>2, only the n=2 shift has been measured so 
far.  That is because the measurement relies on the fact that the 2s(1/2) 
state is "metastable", and lives long enough so that a beam of atoms in 
the 2s(1/2) state can be formed.  (The 2s(1/2) state is metastable because 
there is no lower state for it to decay to with one photon emission.)  So 
that although there is a Lamb shift for all n, it has only been measured 
for n=2.