Handle 4 tables and to show 4 columns as result
First I'll explain the mistakes/errors.

1) UNION provides the vertical merging not horizontal.

+---+---+         +---+---+     +---+---+       
| A | B |         | A | B |     | 1 | 2 |
+---+---+         +---+---+     | 3 | 4 |              +---+---+---+---+
| 1 | 2 |  UNION  | 6 | 7 |  =  | 6 | 7 |          not | 1 | 2 | 6 | 7 |
| 3 | 4 |         | 8 | 9 |     | 8 | 9 |              | 3 | 4 | 8 | 9 |
+---+---+         +---+---+     +---+---+              +---+---+---+---+


Also UNION requires both merged tables have columns of the same 
names/types/order. That is why you get only two columns while the columns and rows from the second table are dissapeared.  

2) That "some syntax error" was the requirement that each tablesource used by query should have the name. When you have used the subquery you should give it an alias:

SELECT SectorTrend.Trend
  FROM ( SELECT Sectors.SectorName
           FROM ( SELECT Companies.Symbol, CompanyData.strenght 
                   --- some missed clause should be here
                   INNER JOIN CompanyData ON Companies.CompanyID = CompanyData.CompanyID
                   WHERE CompanyData.strenght = 'anyvalue'
                ) AS aaa --- that's what I mean
          INNER JOIN Sectors ON Sectors.SectorID = SectorTrend.SectorID
          WHERE Sectors.SectorID = 'anynumber'
      ) AS bbb --- that's what I mean
 INNER JOIN SectorTrend ON SectorTrend.SectorID = Sectors.SectorID 


Anyway that query is too complicated and wrong to be used. 

3) Multiple JOINs
Your mistake is that you have joined the table Companies twice (instead of Sectors) and engine can't tell one instance from another. The good practice is to assign an unique alias to each table in the query:  

SELECT c.Symbol
     , s.SectorName
     , cd.strenght 
     , st.Trend 
  FROM Companies   AS c
  JOIN Sectors     AS s  ON s.SectorID   = c.SectorID
  JOIN CompanyData AS cd ON cd.CompanyID = c.CompanyID
  JOIN SectorTrend AS st ON st.SectorID  = s.SectorID 


I've replaced the INNER JOINs by JOINs because (at least in the mysql dialect) they are exactly the same with ON clause. And this query is my choice for the task.