For investors in emerging-growth mutual funds, the past few months have been a tale of patience rewarded.

Since last fall these funds, which specialize in the stocks of small companies with bright promise, suddenly have surged to the head of the performance derby, after years of struggling along near the back of the pack.Over the four months from the beginning of October through the end of January, small-company growth funds rang up an average total return of 16.3 percent, according to the newsletter The No-Load Fund Investor.

That compared with an average gain of 11.8 percent over the same span for all stock funds.

So far in February, the trend has shown every sign of continuing.

As of Feb. 7, notes analyst John Hoffmann at the investment firm of Smith Barney, Harris Upham & Co., the NASDAQ index of industrial stocks traded in the over-the-counter market - home of many small growth stocks - had soared 40 percent from its October low.

All this has meant a nice, if very belated, payoff for fund shareholders who stuck with their emerging-growth investments while others were reaping almost all the glory in the 1980s.

The obvious question now is whether the turnaround is only temporary or the start of something more substantial.

"Historically," says Sheldon Jacobs, publisher of the No-Load Fund Investor, "small-capitalization stocks have outperformed large-capitalization stocks, but for most of the past seven years the reverse has been true. Their time may be now."

Other analysts, such as Rao Chalasani at Kemper Securities Group in Chicago, are billing 1991 as "the year of secondary stocks."

They point out that small growth stocks have lately been about as cheap as they have ever been, based on one yardstick commonly used to measure their value against that of the overall market.

The typical price-earnings ratio of the stocks in the T. Rowe Price New Horizons Fund, the dozen of emerging-growth funds, late last year dipped below the P-E of the market as a whole - marking the first time that has happened since 1977.

By an old rule of thumb among analysts specializing in this sector of the market, emerging growth is a "buy" whenever the New Horizons P-E is around parity with the market as a whole, and a "sell" when its P-E gets near double that of the market.

Analysts generally still put a lot of credence in this standard. But by itself, they acknowledge, it is no simple key to catching waves of enthusiasm for small growth stocks.

At the end of 1988, for example, it stood squarely in "buy" territory when the group began a rally, only to falter a few months later and lapse back into disfavor.

Analysts at T. Rowe Price Associates, the management firm that runs New Horizons, report that $1 invested in the fund at its inception June 30, 1960, would have grown 30 years later to $16.96.

Of that increase, they found in a study done last year, nearly four-fifths took place in just 12 of the 120 calendar quarters in which the fund has been operating.

This shows, said Steven Norwitz, a Price spokesman, that "the returns in this sector often come in sporadic bursts of performance, which is why investors in small growth stocks must take a long-term approach."

He added: "This sector would be even more difficult to time (with in-and-out trading) than the market on average."