  By means of principal isotopes lH(a,b) of the algebra lH [Ra 99] we give an
exhaustive and not repetitive description of all 4-dimensional absolute-valued
algebras satisfying (x^p, x^q, x^r) = 0 for fixed integers p, q, r \in\{1,2\}.
For such an algebras the number N(p,q,r) of isomorphism classes is 2 or 3, or
is infinite. Concretely 1. N(1,1,1)=N(1,1,2)=N(1,2,1)=N(2,1,1)=2, 2.
N(1,2,2)=N(2,2,1)=3, 3. N(2,1,2)=N(2,2,2)=\infty. Besides, each one of the
above algebras contains 2-dimensional subalgebras. However, the problem in
dimension 8 is far from being completely solved. In fact, there are
8-dimensional absolute-valued algebras, containing no 4- dimensional
subalgebras, satisfying (x^2,x,x^2)=(x^2,x^2,x^2)=0.
