The numeral 4.21 has three significant fire two known figures d e figure numbers are always significant

The significant figures are always:

Different from zero except there are only zeros before the point.

You can round them to the previous significant.

In scientific notation, you have one figure point two more figures.

Examples:

You have 4.21

All different from zero and only two decimals.

Those are all significant figures.

if you have 000.231555

You will shorten this to two significant figures.

Before the point, you only have zeros, so you need to keep only one of them to say its less than one.

After the point, you have a lot of figures, but you need to round this to two.

Because you have a one before the three, you'll keep the three. If you have a five or bigger number, you round it.

In this case, you'll have 0.23 with two significant figures.