Amid the mounting rush to close the Rikers Island jail, the City Council votes Thursday on a scheme to replace it with four borough-based jails with far less capacity. This fantasy “plan” will only make New York City less safe, putting dangerous suspects out on the street. The council’s clear duty is to say “no.”

In a clear sign that politics are driving the design here, City Hall just slashed its projection of the citywide jail population in 2026 (the Rikers-closing year) from 5,000 to 3,300.

The last time it was 3,300 was in 1920, with a city population a third smaller than today’s. There are now just over 7,200 inmates in Rikers, so the re-estimate is from a drop of 30% to one of 55%.

That bit of magic let the mayor slash the size of the proposed replacement jails to ease fears in the target neighborhoods.

Yet it assumes that none of today’s trends in criminal-justice reform will falter over the next six years, including the new state “no bail” law that kicks in Jan. 1.

Thing is, the easy cases are already out of Rikers: Admissions are down by half from five years ago. The jail isn’t packed with turnstile-jumpers, pot-smokers and young men caught in the wrong place at the wrong time: These cases are about guns, gangs and assaults.

Of the current pretrial inmates, 62%face violent-felony charges; 30% face other felony charges, as Rafael Mangual notes.

And most are repeat offenders. Which ones should be walking the streets? Not someone who’ll proceed to kill four sleeping homeless men, or rape a minor child — as did two accused already sprung from Rikers under the existing “clear ’em out” drive.

City Hall is purely focused on overcoming immediate resistance, which is strongest in the neighborhoods targeted for the new (or rebuilt) borough jails. Hence the last-minute magic re-estimate, so that the new lockups can shrink an average of 90 feet apiece.

Another fantasy here is that the city can meet its 2026 timeline for these four construction jobs — all in densely built areas — when its track record for less complex projects (libraries and schools) guarantees that every project will be late. Is that why the mayor won’t commit to closing the Bronx jail barge until 2026, though it holds just 800?

The plan sets the construction budget at $10.6 billion to detain a maximum of 3,300. How is it not wiser to spend the cash remodeling and reforming Rikers itself?

City jails should be safe, clean and violence-free. But neighborhoods (and crime witnesses) need to know that dangerous offenders aren’t walking their streets pending trial. And New York needs to know it has the room to lock up those it needs to — even if crime rates head back up.

Heck, this plan doesn’t even guard against a return to current levels of crime. Will the city just have to go for massive jail overcrowding if things don’t work out perfectly?

Today’s council members will move on before all these fantasies come crashing down. But they still have a responsibility to reject this scheme now — and demand a plan that has at least a chance of working.