We're familiar with the Honda inline-four, actual one of our mechanics has one,

By changing everything that provides cylinder 4 with spark and fuel, you evidently have eliminated those two factors from the equation. But in addition to spark and fuel, an engine also needs air, and that seems to be what is missing. The fact that cylinder 4 will not run at all on the choke and fires weakly once the choke is turned off indicates that it is not receiving the same volume of intake air as the other three. That points to the valvetrain as a prime suspect. If cylinder 4's intake valve is either partially or barely opening, the cylinder would not get sufficient air to support combustion equal to that of the other three; and if the exhaust valve only partially opens, spent gases in the cylinder would have trouble fully escaping, thus contaminating the incoming mixture enough to produce weak combustion.

Although you stated that the compression for cylinder 4 is between 140 and 150 psi, that figure still could be attained with either of the aforementioned valve problems if the engine turns enough revolutions during the checking process. To determine if a valve problem exists, remove all four sparkplugs and the inspection caps for the intake and exhaust valves for both cylinder 4 and 3. Then, crank the engine over while observing any differences in the movement between cylinder 3 and 4 rocker arms. If either 4 valve is not opening far enough to permit reasonably good combustion, it should be easy to spot.

Two conditions could cause insufficient valve opening: a badly worn camshaft lobe or an equally worn rocker shaft – maybe even both. Failures of that type usually are caused by an oiling problem that is restricting adequate flow of lubricant to that part of the valvetrain. Sometimes, however, rocker shafts or camshaft lobes wear out prematurely because of material or surface-hardening problems. Such failures don't happen very often these days but were more common on older engines. Also it's possible that some parts involved are low-cost / low-quality aftermarket parts....

Quote by: Chaidee

I'm also familiar with the Honda CB400F and I can recall that they had to be on the boil to achieve the desired performance. But at a mere 20,000 kilometres, can a cam lobe or rocker shaft on a Honda CB400 wear that much unless the motorbike had been really abused?

Absolutely, I saw several such component failures on older CB400's, and not all were the result of what could be termed 'abuse.' Sometimes, a cam lobe or rocker has escaped the factory without sufficient surface hardness. On a couple of occasions, the problem occurred due to a low oil level caused by a small, undetected oil leak. Other times, it was the result of the owner not changing the oil and filter often enough or just changing the oil but not the filter.

I guess you could call some of those factors abuse, although back in the days, most motorcycles were more susceptible to even slight tardiness in performing scheduled maintenance than are today's motorcycles. But even the most sophisticated of current motorcycles still have warranty issues, proof that they remain imperfect machines manufactured by imperfect human beings.