Hello
There is no need to 1st upgrade to Ehp3 for upgrading to Ehp4.
Using Ehpi (at that time) or SUM (now) you can directly upgrade from ECC6 to ECC6 Ehp4.
Ehp are new functionality added to the SAP system but Ehp are not cumulative.
Stackfile defined by the maintenance for upgrading from Ehp-x to Ehp-y does include the files for the target Ehp y + for the Ehp in between x and y.
Upgrading from Ehp4 to Ehp7 can also be done in one pass.
You can read the great document from Boris Rubarth on that subject
=> SAP versioning, part 2 (why EHP7 contains SAP APPL 6.17)
Regards