The Studio IV uses some kind of extended CDP 1864 for video output. Main difference from the CDP 1864 is the fact that the Studio IV chip displays a pattern of 128x64 pixels instead of the max resolution of 64x64 of the CDP 1861 and 1864.
No EF line is used to indicate display status, instead it looks like OUT 5 (and 7) is used to signal the video chip to enable DMA towards the 1802 for fetching display data.
The Studio IV controls the video chip via OUT 4. Bit 0 to 2 select one of 8 background colours, bit 3=1 forces all foreground colour to white, bit 4 and 5 (both 1) switch the display on and bit 6 looks like a switch between NTSC and PAL mode.
The Studio IV firmware uses the resolution of 128 x 64 with interrupt routine entry point at @004F. The system ROM defines 2000-23FF as the display RAM and 2800-2BFF as the colour RAM (4 bit max). Assumption is that like the 1864 only 3 bits are used for 7 colours.