repeated measures using lmer

I am trying to run a linear mixed effects model with the following groups.
My DV is reaction time and fixed factors are time (pre vs.
post:within-subject), condition (congruent vs. incongruent: within
subject), SOA (early vs. late: between subject) and stimulation (vertex vs.
DLPFC: between subject).
My concerns are:
a) I have very few participants in each SOA condition: the early SOA
condition has only 3 participants in the vertex and 4 in the DLPFC condition
while the late SOA condition has 2 participants in the vertex and 4 in the
DLPFC condition

b)2 out of the 4 participants  in the late SOA condition participated in
both vertex and DLPFC condition.

is it possible to compute a lmer with such few participants and how do i
account for random intercepts and slopes with few number of participants?
Thanks for all the help