Yes it wont make a difference to your GPA, but it will affect how your GPA will be considered.
Like i said before the GPA is just the weighted average of your grades. But uni's will turn GPA's from different durations of study into different selection ranks - which is what they will actually use for entry.
Have a look at this document from UQ for example. You can see that a GPA will convert to different ranks depending on how many semesters of study you do.
E.g. If we look at how a 6.5 GPA will be considered into a selection rank
| 0.5 semester | 1 semester | 1.5 semesters | 2 semesters (1 year) |
| 85.00 | 88.00 | 94.00 | 99.00 |
If you did the 4 units throughout the year (e.g. 1 semester EFSTL) a GPA of 6.5 would convert to only an 88, which wouldnt nearly be competitive for entry into Adelaide. This disadvantage by far overcomes any advantage you would have by taking a lower work load.
Considering from A1's post he points out that Adelaide just takes selection ranks from SATAC and then does the caluclations from there... you would be best of emailing SATAC and asking them if a gpa from less than 1 EFSTL would convert less favourably than one from more than 1 EFSTL (i would bet a large amount of money on the answer being a yes)