Where's my missing Sql Server Express 2005 Database space? Size and space available dont add up to 4G?
On the question of the size of things, first of all you said that you shrank a database, so that would reduce the DB size of the space that was discarded during the shrink.  John M also provided some clarification.

Regarding the numbers in the bottom picture, they are:

select (2585832 / 1024.0) /* KB from the Reserved in the bottom picture to MB */ 
       + 43.96 /* MB free */ 

SpaceAllocated
---------------
2569.186562


So you can see that 2569.19 MB is essentially the same as 2569.73 MB

The other numbers in the bottom picture are showing you how that database is allocated:

reserved        data         index_size  unused 
-----------     ----------   ---------   --------- 
2585832 KB   =  2315184 KB + 137216 KB + 133432 KB


So it is pretty straight-forward and actually gives you some useful insight into your space utilization.

Additional Notes: Yes, when you shrink a database, it actually compacts the data and then releases the no longer used disk space.  Of course, SQL Server 2005 Express can have many databases that are 4 GB.  (If you update to a more recent version of SQL Server Express, the limit is 10 GB per database, not including the log space.)

Within a database the unused column in the example, defines how much space currently exists in the database file that could be used for adding more data.  However, the database can be expanded to its maximum size or you can set autogrowth to allow it to grow in selected increments. (Use exact growth values, not percentages.)