Basically, the 4000 Series is the low end single motor deck. The GX-4000D was one of the last Akai's made, and is the only Series 4000 unit which has the GX Heads. These sets use a single speed motor. To go to the slower speed or the faster speed, on those sets, it is done manually and mechanically.
If the rotating capstan has a knurled cap in place, it is retaining the thin metal CAPSTAN SLEEVE, and the set is then operating at 7 1/2 ips which is the faster speed. These items must be manually removed to change from the faster 7 1/2 ips speed to the slower 3 3/4 ips speed. If the set is running at the slower speed, the capstan will show a small hole in its center (to be used by the knurled retaining cap after you put the capstan sleeve on first).
The Capstan Sleeve must be taken and placed onto the capstan and retained using the Knurled Cap ( which has a reverse thread ) to go to the faster speed. Speed change is easily done when the set is turned off, or the takeup tension arm is relaxed into the down position, causing the motor to stop. To switch to the slower speed, unscrew the knurled cap - do so counter-clockwise. Then lift and remove the capstan metal part, and then place this capstan sleeve on the hitching post, which is present either above the headcover ( as on the 4000DS or 4000DB ) or under the head cover ( as on the GX-4000D, the head cover pulls straight off, and the hitching post is next to the black erase head ). The hitching post is also threaded so the knurled cap will attach to keep the sleeve in place. Don't lose either piece! These capstan sleeves and caps are impossible to duplicate or machine up, so far, at least, no-one has been able to do it. The EQ Switch should be then set to change the playback amplifier equalization to match the speed you have chosen to use.
Sets in the M series and 1700 series also use capstan sleeves to initiate a speed change, but they are mechanically somewhat different parts and are NOT interchangeable, and those other sets don't use the retaining cap, and so they also don't have a hole in the center of the capstan. Within the 4000 Series, a number of sets are mechanically very similar, all with the two- knob mechanical operation block. There were various electrical differences, some had different motors, some had permalloy heads, while later sets had the GX Crystal Ferrite type, some had large VU meters, all 4000 series sets were transistor, only the early M Series or 1700 / 1800 series sets had vacuum tube electronics.