Type 1A4 is a 1-series vertical plug-in with four input channels and 50MHz bandwidth. The four channels are grouped into two groups of two channels. Type 1A4 has a special "trace slaving" mode which, when used in a 547 mainframe, allows channels 1 and 2 to be displayed using the A time base, and channels 3 and 4 to be displayed with the B time base. This allows high-speed and low-speed signals to be displayed together, similar to what a dual-beam scope can do, but without some of the cost and complexity. Maximum channel sensitivity is 10mV/div, but a Signal Out jack makes it possible to cascade two channels for approximately 200uV/div AC with about 20Hz-1MHz bandwidth. Like the Type 1A2 and unlike Type 1A1, Type 1A4 has a Trigger Output selector that controls which channel provides triggering when the mainframe trigger is in Plug-In mode.
The signal path of each channel of the 1A4 is as follows. First the signal passes through the attenuators. Then it is clamped by two diodes and applied the the gate of a JFET source follower. The source of the source follower is the trigger pickoff point. From there the vertical signal path goes through an NPN emitter follower followed by an NPN differential amplifier, at which point the signal becomes differential. Next, the signal passes through an analog multiplexer made with 1N4152 silicon switching diodes. After the multiplexer network, the signal passes through a common base amplifier followed by 6DJ8 tube operating in common grid configuration. Type 1A4 was introduced in 1967 and lasted until the end of the classic plugin scope line in 1974.