For reasons discussed in another topic pertaining to the inequities between Lannister and Greyjoy:
Lannister is not used in 4 or 5 player games. Tyrell and then Tyrell and Martell are used instead (respectively)
IF you insist on using Lannister: (listed in order of most severe effect to board layout and army strengths)
1. Lannister starts with 2 footmen in Lannisport and 1 knight in Riverrun. (high)
2. Lannister starts with 2 footmen and 1 knight in Lannisport. (moderate)
3. Riverrun has the Martell 4 player (5 strength) garrison on it. (light)