There is no such thing like a CB400/4 on Honda listing before after 1977 in US. It had 17teeth on the front sproket and 38 on rear sprocket. It uses a standard 530 chain. Many third party drive chain and sprocket kits are available.
Reducing the number of rear sprocket teeth or increasing the number of teeth on the front could theorically increase top speed at the expense of acceleration power, if there is enough torque left to drive such top speed... I doubt it. A FOUR is already an acceleration devil but at top RPM, there is only so much you can expect from an engine with two valves by cylinders. You already have SIX speeds on your transmission to extract the best of your available power curve. This bike is rated for max practical speed of 175km/h on sixth speed out of a 37HP engine !
I've got a Honda CM400T 1980 twin engine with 43HP and three valves per cylinders with a five speed transmission. It is said to reach 156km/h. Just follow me... in 1982 they came up with the CM450 wich had two valves per cylinders and SIX speed transmission. Top speed was NOT improved over CM400.
You are already at very high top speed for a 400cc from the very design of this four cylinders machine. Increasing it would require more than playing with teeth numbers. It would be a shame to ruin such a roarring gem trying to ride it in the red for too long.
Unless you trust a pro mechanic specialized into racing machines, I just would not even try to improve the performance of your CB400F. This is likely a waste of money. You could rather check for foam air intake, modified jet sizes and tuned exhaust. You are talking of big bucks for an expected improvement that is not likely to exceed a few points of percentage, if your engine and driver can survive long enough to test it. Do you really want to ride a 32 year old motorcycle over 175 km/hour !!!??? This could only be acheived if compression is top shape and nothing is out of optimal adjustment, perfect tires, wheels, etc.
