The 4Big units were similar to the 4CIG units but contained a buffet car in place of the intermediate trailer. These units were built in two batches. 'Phase 1' units were built in 1965/66 and were numbered 7031-48. 'Phase 2' units were built in 1970 and were numbered 7049-7058. The 4Big fleet were initially classified as Class 420 by British Rail under the TOPS system introduced in 1968, and then Class 422.
Only two units have been preserved, in addition numerous buffet cars from 4Big units have also been preserved