Problem 3:

Any of the four general subproblem forms would work.  I think
the first two are easier to work with.

Option 1: The general subproblem is to find the cost of
the best solution from post i to post n.  The answer
for the original problem is when i=1.

Option 2: The general subproblem is to find the cost of
the best solution from post 1 to post i.  The answer
for the original problem is when i=n.

Option 3: The general subproblem is to find the cost of
the best solution from post i to post n where you 
cannot exchange your canoe until at least post j.
The answer to the original problem is when i=1
and j=2.

Option 4: The general subproblem is to find the cost of
the best solution from post 1 to post i where the
latest post when you can exchange your canoe
is post j.  The answer to the original problem is when
i=n and j=n-1.

============================================================

Problem 4:

Either of the follwoing two general subproblem forms
will work.  You can also create variants where you
create some non-boolean cost but there is no need to
do this.

Option 1: The general subproblem is to determine
if x[i],...,x[m] and y[j],....,y[n] can interleave
to form z[i+j],...,z[n+m].  The answer to the
original problem is when i=1 and j=1.

Option 2: The general subproblem is to determine
if x[1],...,x[i] and y[1],....,y[j] can interleave
to form z[i],...,z[i+j].  The answer to the
original problem is when i=m and j=n.