Verse 3-4

3-4. Higher than heaven,  seated in the intellect of the heart That shines which the self-controlled attain.  The Ascetics,  who being of pure minds have well ascertained the Reality,  by the knowledge of Vedanta,  and through sannyasa.  In the sphere of creation,  at the time of cosmic dissolution,  they will have achieved liberation.

"Higher than heaven", 

Heaven and all relative terms refer to this apparent Creation of duality which is transient and mortal.  "That" which "shines" referred to here is That Absolute Knowledge which is eternal because it is Primal knowledge and transcends this Creation.  It is immortal therefore "Higher than heaven".

"seated in the intellect of the heart"

This verse contains a profound truth of Advaita,  of non-duality,  The meaning here is that Absolute knowledge is Eternal Existence and is therefore eternally available,  and can be attained through the intellect or Buddhi purified through right practices.

"That shines which the self-controlled attain"

"That" which shines is the "self" which is eternal knowledge and Consciousness that is the Absolute Self.  Although this knowledge is available to all and is seen by all it is the "self-controlled",  meaning those having undergone a discipline of yoga and meditation,  that become more conscious of "self",  of the Truth of this subtle knowledge.  Those who believe the material world to be the "real world" rationalise this insight away too quickly.

"The Ascetics,  who being of pure minds have well ascertained the Reality,  by the knowledge of Vedanta,  and through sannyasa (Final Ashram practice)".

The Kaivalya Upanishad here describes the Ascetic having undertaken the path of the Sannyasin.  Through contemplation,  meditation and complete disentanglement from the sensual and material world,  the intellect of the heart,  Buddhi,  has been purified enabling proper discrimination for the truth of the "self" to be intuitively seen,  This truth then becomes the proper guide enabling the realisation of the Absolute Self.

"In the sphere of creation,  at the time of cosmic dissolution,  they will have achieved liberation".

This verse tells us that this realised being achieves liberation (unity with the Absolute) in this sphere of Creation,  this lifetime or,  by the time of the end of material Creation,  "Cosmic dissolution".

This means it is through the teaching of the sage and meditation practices that one purifies the intellect that can intuitively discriminate higher knowledge to Realise the truth of Existence that liberates one.