4th is regular... it will be like 0(0+1)*0 + 1(0+1)*1 + 0 + 1 + epsilon

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it is regular language

as language consists of equal number of 10 and 01

Now, suppose in the string you have 01 , now we need another 10 to make count same

this can be thought of as whenever you see a 01 at some point next character should be sequence of 0 or a single 0. in that case string formed is of the type 010*0. therefore, the count of 01 and 10 remains the same. But if you have 011 type sub string then you need to see another 0 for example 011*0 is also a valid string.

so the regular expression is 1(1+00*1)*+0(0+11*0)* + epsilon