PostgreSQL does not choose my indexes well - postgresql

Question

create table public.tabla
(
cod_tabla bigint not null,
tabla varchar(31) not null,
constraint pk_tabla primary key (cod_tabla)
);
create table public.entidad
(
cod_entidad bigint not null,
cod_tabla bigint not null,
cod_entidad_tabla bigint not null,
constraint pk_entidad primary key (cod_entidad),
constraint fk_tabla_entidad foreign key (cod_tabla)
references public.tabla (cod_tabla) match simple
on update cascade
on delete cascade
);
CREATE INDEX idx_tabla_entidad
ON public.entidad USING btree
(cod_tabla ASC NULLS LAST);
CREATE INDEX idx_entidad_tabla_4
ON public.entidad USING btree
(cod_entidad_tabla ASC NULLS LAST)
INCLUDE(cod_entidad, cod_tabla, cod_entidad_tabla)
WHERE cod_tabla::bigint = 4;
I think postgresql doesn't use the index idx_entidad_tabla_4,
Postgresql is sequentially scanning the entire table applying the where condition
explain (analyze, buffers, format text) select * from entidad where cod_tabla = 4
Index Scan using idx_tabla_entidad on entidad (cost=0.56..51121.41 rows=1405216 width=20) (actual time=0.037..242.609 rows=1409985 loops=1)
Index Cond: ((cod_tabla)::bigint = 4)
Buffers: shared hit=12839
Planning Time: 0.158 ms
Execution Time: 311.828 ms
SELECT count(*) from entidad;
34.413.354
SELECT count(*) from entidad where cod_tabla = 4;
1.409.985
My questions are:
Why doesn't it use the index idx_entidad_tabla_4?
How could I force its use?