Compatibility issues between FastReport 3 and FastReport 4:

- v4 uses the same file/package names as v3. You have to uninstall v3 before 
installing v4.

- v4 uses the same file format, .fr3 and can open v3 files without any problems.
Most of new report files also can be opened by v3, but not all.

- v4 can read v3 .fp3 files.

- New kind of page, "Data" page is introduced in v4. This page contains all
internal datasets of your report. When opening old v3 files, v4 automatically 
adds the "Data" page and moves all datasets on it.

- Since all v4 reports contain a "Data" page, you should check your Delphi 
code where you access a report page by index. The "Data" page is always the 
first one. So code like Page := frxReport1.Pages[0] will return a data page.
You should correct that code (either use [1] or access a page by its name).

- Check your reports that contain the Cross-tab object! 
a) In v4, this object draws cross-tab elements in design-time 
and is bigger than in v3. So you have to correct bands that contain
such object.
b) New behavior introduced in v4 - table can have corner, title, cell headers.
All these properties are on by default. You may turn off corner and title
(ShowCorner, ShowTitle properties) if not needed, and fill in the cell 
headers elements.
c) In v4, table has several cell elements. Each cell element has its own
font/color/frame/... settings. In v3 you were able to setup only one cell 
element. You should set the fonts and colors for each new element, if needed.

- frxHiButtons unit is no longer used - remove it from your "uses" list