Switching from indexed to sequential scans

Previously, it was pointed out that when the query planner knows much of the table is going to be returned, it will just execute a sequential scan. This still isn't the case even in the very poor plan that is given, if we want all the values from 0 to 3 here:

EXPLAIN ANALYZE SELECT COUNT(*) FROM t WHERE v<4;
QUERY PLAN
----------
Aggregate  (cost=1706.53..1706.54 rows=1 width=0) (actual time=112.030..112.031 rows=1 loops=1)
   ->  Bitmap Heap Scan on t  (cost=665.33..1606.83 rows=39880 width=0) (actual time=5.712..59.564 rows=40144 loops=1)
         Recheck Cond: (v < 4)
         ->  Bitmap Index Scan on i  (cost=0.00..655.36 rows=39880 width=0) (actual time=5.609..5.609 rows=40144 loops=1)
               Index Cond: (v < 4)

But, even with the relatively small amount of data we're giving the planner (more on that in a while), if you ask for more than that it realizes a sequential scan will execute better:

EXPLAIN...