I think with a systematic approach which I will describe below, you could achieve the level where you literally sight-read any hymn you want in four parts with pedals. The end result would be like this - soprano and alto would be played by the bass, tenor - by the left hand, and the bass - by the pedals with 16' stop as a foundation.
This is of course the most common disposition of parts. If you want to be more creative in church, read this article.
So in order for this process to be as smooth as possible, here is a path you could follow:
1. Soprano part with the right hand
2. Alto part with the right hand
3. Tenor part with the left hand
4. Bass part with the pedals
5. Soprano and alto with the right hand
6. Soprano and tenor with both hands combined
7. Soprano and bass with right hand and pedals
8. Alto and tenor with both hands combined
9. Alto and bass with right hand and pedals
10. Tenor and bass with left hand and pedals
11. Soprano, alto and tenor with both hands combined
12. Soprano, alto and bass with right hand and pedals
13. Soprano, tenor and bass with both hands and pedals
14. Alto, tenor and bass with both hands and pedals
15. All parts together with both hands and pedals
NOTE: Each step should be practiced with different set of hymns from your hymnal.
If your goal is to learn hymn playing, try the above approach - it really works. For some people it will take about a year, for some 6 months, and for some - only 15 weeks.