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Greetings!

The smallest number in the series is 1000 which is a a 4-digit number while the largest number in the series is 4000, the only 4-digit number to start with 4.
Now,the left most digit (thousands place) of each of the 4 digit numbers other than 4000 can take one of the 3 values 1 or 2 or 3. Thus ,the left most digit have values 3. The next 3 digits (hundreds, tens and units place) can take any of the 5 values 0 or 1 or 2 or 3 or 4.
So, The next 3 digit have values 5.

Then ,
there are 3*5*5*5 numbers = 375 numbers from 1000 to 3999.

Now,on including 4000,
there will be 376 such numbers .


thankyou