How to add missing sequence number in SQL server table

The principle is that you need to select a contiguous list of numbers and then match them to the data you have in your table.

Here is your data

declare @demo table ([week] int, [year] int, platformname nvarchar(30), operatingsystem nvarchar(30))
insert into @demo ([week], [year], platformname, operatingsystem) values
(1, 2018, 'sql', 'w7'),(2, 2018, 'sql', 'w7'), 
(5, 2018, 'sql', 'w7'),(6, 2018, 'sql', 'w7')

As you say there is a gap where 3 and 4 should be.

Here is a neat and efficient way of generating all the numbers you want

select * from 
(select top (@top) row_number() over (order by  a.object_id) AS seqNum 
from sys.columns a) as nums

That query uses the system table sys.columns just because I know there is going to be some data in there, probably quite a lot depending on my database schema. In my case there are 1167 columns in my database, but if that isn't enough you can use a cross join to the same table to get even more numbers - 1361889 in my case i.e. 1167 x 1167

So I didn't end up with 1361883 empty rows in my results I've limited the number of numbers I want by using top (@top) where

declare @top int = (select max([week]) from @demo)

I could just as easily have hard-coded this to 10 for example.

Combine the two queries with a LEFT OUTER join using the numbers table as the left table and your data table as the right table i.e.

select nums.seqNum, [year], platformname, operatingsystem
from 
  (select top (@top) row_number() over (order by  a.object_id) AS seqNum 
  from sys.columns a cross join sys.columns b)  as nums
left outer join @demo  on seqNum = [week]

which gives the results

seqNum	year	platformname	operatingsystem
1	2018	sql		w7
2	2018	sql		w7
3	NULL	NULL		NULL
4	NULL	NULL		NULL
5	2018	sql		w7
6	2018	sql		w7

You'll have to decide what to do with the NULL values