Complex sentence: Most of the 4th Marquess's art collection had been acquired by himself or his father, and went to Wallace, and is now the Wallace Collection, but other works were covered by the entail, and passed to the 5th Marquess.
Split into simpler sentences: Most of the 4th Marquess's art collection had been acquired by himself or his father, went to Wallace, and is now the Wallace Collection. Other works were covered by the entail, however, and passed to the 5th Marquess.