JUST a year ago, it would have seemed unlikely that four of the world's best-known tech firms - IBM, Hewlett-Packard, Facebook and Yahoo - would have women in, or near, the top job.

But four capable, smart and ambitious women do now fill senior roles at these global giants.

Ms Sheryl Sandberg, 43, has been chief operating officer of Facebook since she joined the social network in 2008.

Last September, Ms Meg Whitman, 56, was promoted from board member of Hewlett-Packard to be chief executive.

Ms Virginia Rometty, 55, took the reins as IBM CEO in January this year, the first time the 101-year-old company has been led by a woman.

And on Monday, Ms Marissa Mayer became CEO at Yahoo. This is an unusual appointment. At 35, she is the youngest woman - and maybe one of the youngest executives - to make CEO.

Together, the four are running not only well-known brands but also substantial businesses whose total market value is a huge US$328 billion (S$413 billion).

Their appointments should cheer women everywhere because it shows that they can succeed in the male-dominated tech world. It should also encourage girls everywhere, including in Singapore, to follow in the footsteps of these women who have degrees in engineering, maths and computer science. Tech companies always prefer their executives to have these backgrounds.

Once Ms Mayer's appointment becomes old news, the spotlight will turn to whether she and Ms Whitman can turn their beleaguered companies around.

I am taking a five-year view.

In five years, Ms Rometty will probably retire, as IBM CEOs seem to take 60 as the mandatory retirement age. So in 2017, when Ms Rometty retires, will the two other women still hold their current jobs?

The next step up for Ms Sandberg is to become Facebook's CEO. That should be a challenge as the social network's co-founder Mark Zuckerberg shows no intention of stepping down.

She may jump at an opportunity to lead a company or even form a start-up of her own.

Likely scenario: By 2017, she will no longer be at Facebook.

The outlook for Ms Whitman and Ms Mayer is equally uncertain. The problem lies not with their ability for they are smart, capable and well-networked in Silicon Valley. But they have to fix broken, rudderless companies, the result of too many leadership changes. And they have to do this in a fast-moving, competitive market. Their rivals have already stolen top talent and eroded their market share.

Broken companies need time to mend. Any strategy would need at least two to three years to show some results.

But Wall Street is not the only one impatient for results. So too are the company boards, shareholders and tech pundits who would want to see tangible results to help restore HP and Yahoo's reputations as innovative companies and tech leaders.

The two women are unlikely to get the two to three years they need to do this.

Likely scenario: By 2017, both Ms Whitman and Ms Mayer will not be in their current roles.

This will be sad as it will quicken the decline of HP, the grand daddy of the IT industry and Yahoo, an early Internet leader.

If there is a lesson to be learnt, it is this: It does not matter if leaders are smart and capable, whatever their gender. Leaders who fail to get their companies to innovate with new ideas and to turn out products to generate profits every three months will have to go. This is the brutal reality of the tech industry.