Volume 4, first published in 1919, contains systematic descriptions of fossil ginkgoales and coniferales.
40. Ginkgoales; 41. Genera believed to belong to the Ginkgoales but which on the available evidence cannot be referred without hesitation to that group; 42. Genera of uncertain position; 43. Coniferales (recent); 44. Coniferales (fossil); 45. Coniferales (continued); 46. Coniferales (continued); 47. Coniferales (continued); 48. Coniferales (continued); 49. Coniferales (continued); 50. Coniferales incertae sedis; 51. Podozamites and Nageiopsis: genera incertae sedis; 52. Gnetales; Bibliography; Index.