Let us say the value of the market now is M.

You have 4% of M. That means you have (0.04)M.

The M grows by 10%. So the new market is (1.10)M.

But your share only grow by 8%. So your new share is (1.08)(0.04M) = (0.0432)M

What is your new market share?

New market = (1.10)M.

Your new share = (0.0432)M.

What percentage of (1.10)M is your (0.0432)M?

It is

(0.0432M / 1.10M)(100%)

= (0.0432 / 1.10)(100%)

= (0.03927)(100%)

= 3.927 %

Your share of the new market is 3.927 percent only.

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A formula?

Let A = your old share, in decimals

And B = the growth of the market, in decimals

And C = your share's growth, in decimals

And D = your new share, in decimals

D = (1+C)*A / (1+B)

D*(100%) = your new share in percent.

Try that on your question above.

A = 0.04

B = 0.10

C = 0.08

D = (1+0.08)(0.04) / (1 +0.10) = (1.08*0.04) / (1.10) = 0.03927

In %, D = 0.03927 *100% = 3.927 %