If you understand accumulator as the result register of ALU, this must have 4-flip-flop for 4-bits (typically), even in pipe-line. However, other regiters in pipe-line architecture, such as ALU inputs, may hardwire 'forwarding' technique in order to avoid 'pipeline hazards' as data dependence. Pipeline architecture has big size register to store control and data, while they come across functional stages.