DIfferent condition in each run

Hi,
In our task design subjects have 4 runs for two sessions (one session per day).
Each run is a block in which they undergo the same task basically.
I have four conditions Gain with risky choices, Gain with ambiguous choices and Loss with each of the mentioned (risk and ambiguity).
Each run will present either gain or loss. So basically I have two runs(blocks) of gain and 2 of loss per session.

How can I set up a fsl analysis to contrasts all of them?
I thought I should add empty EV (i.e. [0,0] in onset and duraion) for the “missing” conditions in each run but that didn’t work.

Any ideas?

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