Editor's note: This column was written prior to the May 4 debate held at Temple University.

THE MAYORAL candidates will meet tonight for a debate sponsored by the Next Mayor project, and it should be a lively exchange. You can catch it at Philly.com/debate.

However, I'm willing to bet that the following topics won't come up - at least in any detail. Combined, though, they represent the greatest challenges that will be faced by the next mayor. And, unlike the schools, all of them deal with departments and policies directly under the mayor's control.

Here are the four, in rough order of their importance:

Pensions. Warren Buffett has called the issue of public pensions a huge explosive with a long fuse. In Philadelphia, the fuse is getting shorter. The city's pension fund has a long-term deficit in excess of $5 billion. The city has only 47 cents in the fund for each dollar it will eventually have to send out to retirees.

At least Mayor Nutter tried. He opted for an asset sale - of Philadelphia Gas Works - but City Council wouldn't even debate the idea.

The Pennsylvania Intergovernmental Cooperation Authority, the state board that oversees the city's finances, recently issued a staff report suggesting ways to lower its deficit, but many are politically unpalatable: abolish the Deferred Retirement Option Plan, increase employee contributions, require that employees go into a "hybrid plan" that includes a 401(k).

None of the candidates has embraced these ideas.

Why it's important: State law requires local governments to make what are called MMOs - minimum municipal obligations. To put it bluntly, the city's pension MMO means the more the city must pay into the fund, the less that can go into police, sanitation, recreation and other city services.

Policing. There's been talk about "stop and frisk" in this campaign, but that is just part of a larger issue that centers on how best to deploy and train police and how they interact with the community. For 20 years, law enforcement has been guided by two principals: use of data to help fight crime, and the "broken windows" theory that says that police have to keep an eye on small items - a broken window, turnstile hopping, petty theft - to prevent growth in crime overall. Stop and frisk is part of that preventive strategy.

The police - including Commissioner Charles Ramsey - believe these methods have resulted in a decease in crime, and the numbers of crimes large and small have gone down in the city. Despite all the talk of community policing, critics believe that these crime-fighting policies have alienated many in minority communities because they end up unfairly targeting black men.

Why it's important: The next mayor and his or her police commissioner are going to have to decide on a crime-fighting strategy. Ramsey believes in the current methods, though he admits they can be tweaked by such things as better training. He wants the police to be seen as community partners, not as an occupying force. It's a good goal, but wishing doesn't make it so. As we know from other cities, the situation can become volatile. The next mayor needs to reconcile the police and poor communities - without risking a rise in crime.

Pent-up demand. The recession battered the city and its employees, but the unions mostly blame Nutter for failing to address their needs. The hostility - among police, firefighters, blue- and white-collar city workers - is palpable. They feel they have been shortchanged and dissed for the past eight years. Contracts with all city unions have to be negotiated in the next mayor's first term. The unions want to feel the love.

Why it's important: Pay and benefit increases, plus work-rule changes, all have price tags. In a city where growth in tax revenue is incremental - at best - a mayor has to balance the needs of employees with the needs of city residents. Maybe it was a question of style - Nutter and the unions simply didn't mix. Can the next mayor say "No" without engendering the same hostility? Can he or she say "Yes" without harming the city budget?

Conflict over growth. The good news about the city is that its population is growing, changing neighborhoods - and housing values - in positive ways. But the dilemma, as housing economist Kevin Gillen put it: "People want growth but not change." Although newcomers believe they are revitalizing neighborhoods, old-timers see it more as an invasion. As housing prices rise, so do taxes. Rents become pricier. Older residents get displaced by "progress."

Why it's important: The alternative to growth and renewal is stagnation and decline. The next mayor will have to find a way to make peace between the old and the new. He or she can do that through personal leadership and by policies on housing and redevelopment that take the old community's needs into consideration. It's a difficult balancing act that will require political skills and smart policy.