The unbalanced equation:

P₂H₄ ---> PH₃ + P₄H₂

Eliminate all hydrogen:

P₂⁴¯ ---> P³¯ + P₄²¯

Break into half-reactions:

P₂⁴¯ ---> P³¯

P₂⁴¯ ---> P₄²¯

Balance each half-reaction:

2 e¯ + P₂⁴¯ ---> 2 P³¯

2 P₂⁴¯ ---> P₄²¯ + 6 e¯

Make electrons equal:

3 [2 e¯ + P₂⁴¯ ---> 2 P³¯]

2 P₂⁴¯ ---> P₄²¯ + 6 e¯

Add the two half-reactions together:

5 P₂⁴¯ ---> 6 P³¯ + P₄²¯