[Imo]

In the he hydrogen atom, how many emission lines would be observed considering only transitions between the four levels?
http://imageshack.us/photo/my-images/826/semttulohe.png/

I do not understand what the question wants...should the answer be 4? because 1 line of transition =  1 level = 1 emission line .

[Ari Ben Canaan]

There are only 4 spectral lines in the VISIBLE region.

See this : http://www.chemguide.co.uk/atoms/properties/hspectrum.html

[cheese (MSW)]

The  Rydberg Eqn states:
1/λ  =  R[1/n1^2  - 1/n2^2]
Where λ is the wavelength of the light;
R is the Rydberg constant:  R = 1.09737x 10^7 m^-1
n1 and n2 are integers such that n1 < n2 
Consider n2 = 4; possible transitions are 4→3, 4→2, 4→1
Repeat logic for n2 = 3 and n2 =2.
Check ref 3; note only transitions to n1 = 2 are in the visible.
[1] http://en.wikipedia.org/wiki/Rydberg_formula
[2] http://en.wikipedia.org/wiki/Rydberg_constant
[3] http://en.wikipedia.org/wiki/Hydrogen_spectral_series
[4] http://www.chemguide.co.uk/atoms/properties/hspectrum.html