st: Carry forward an observation within a time frame

["Benigno Rodriguez, MD" <benignor@gmail.com>]

Dear Stata listers:

I have a panel dataset that consists of multiple visits on multiple 
subjects at some of which a CD4 value is obtained. My objective is to 
carry forward the CD4 value to visits for which it is missing, but 
only if a value is available within the 4 months prior.

I recognize the problem as one of spells, and have read Nick Cox's 
excellent article on spells from 2007, as well as his column on lists 
from 2002 (which I recognize as even more relevant to my problem), 
but despite his heroic efforts at a foolproof introduction to for and 
its variants, programming does not seem to get through my thick skull 
easily. Could I get a hand with this, ideally not involving 
code?  Below is a relevant excerpt of the dataset, with the desired 
result in the last column.

Thank you very much in advance.

patid	date		CD4	desired
1007	5-May-55	.	.
1007	1-Jan-00	.	.
1007	3-Apr-02	5	5
1007	8-Apr-02	.	5
1007	11-Apr-02	.	5
1007	13-May-02	.	5
1007	14-May-02	4	4
1007	17-Jun-02	9	9
1007	12-Nov-02	.	.
1007	27-Jan-03	6	6
1007	17-Mar-03	.	6
1007	14-Apr-03	0	0

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