If I'm reading this correctly, at every 4th level, you want the character to pick either +1 to strength, or +1 to constitution. However, if they picked +1 Strength for their standard ability increase, they can't pick strength for their third; similarly, if the picked +1 Con, they can't pick Con for their third.
Firstly, what happens if the character picks +1 Str and +1 Con for their 2 standard ability increases?
Secondly, the best way is probably to do the following:
1) Firstly, create a "+1 to Str or Con" race feature, that works a lot like the standard +2 pick.
2) In the editor, use the "Bootstraps" button to add it to the race, 7 times.
3) Set up a condition (using the "Condition" button) on each bootstrap like:
That one will, for example, only show the ability when your level is 4 or more. Use a priority of around "Setup/100".
Now, a new copy of the ability should appear every 4th level that you add to the hero.
Does that work?