I have a value stored in a variable. How can I write this value to a table (for example into Row2 Column4) using Mapbasic?
First you need to select the row you want to update:
Select * from MyTable where MyColumn = "MyData" into ToUpdate
Then update the selection with the data from your variable, in this case
Update ToUpdate set Col4 = varMyValue
Alternatively, you can update by RowID although that is not recommended as this can cause problems when a table hasn't been packed etc, so it is better to select the specific row you want to update first. If you do want to update by RowID it is just a slight alteration to the update statement, but again, it is best to avoid this:
Update ToUpdate set Col4 = varMyValue where RowID = 2
I would highly recommend consulting the MapBasic Reference Guide (available online, just google it) and looking specifically at how the Update statement works. It is also a great reference to get to know all of the available commands in the MapBasic language.