4 Which is the difference between subjective and objective knowledge?

This is a well known dichotomy, although its sense remains somewhat vague, close to the layman dualism between opinion and truth. The adoption of a neural viewpoint seems fruitful here, too, in order to better define the question; two different pathways can be outlined, related to one and the other term.

The first pathway is quite ancient. Its origins can be traced back to the development of simple neural aggregates capable of processing incoming sensory input in ways suitable to induce appropriate responses. Soaring of complexity along different evolutionary lines occurred later, with the development of increasingly refined mechanisms - guided by a simple adaptive logic.
Relationships with neural conscious instances could be established - if these latter were present - when the level of complexity had become sufficiently high to make the connection useful. In humans, only the final results of implicit processing attain the conscious level, in intuitive, synthetic form, possibly associated with generated characters named qualia.
Qualia do not exist as such in the external world, and do not add any new information. Their apparent simplicity is often the result of complex processing (like, for instance, the generation of a variety of colours from electromagnetic waves, or of the third dimension from a flat stereogram), with the possible meaning of easing prompt utilization (in the same way as false colors are used to enhance graphic representations) and of connecting with the emotional sphere. Qualia cannot be translated verbally, but can be shared with other individuals, to a certain extent, through empathy.

The second neural pathway of knowledge, much more recent, is characterized by the development - pragmatistically guided in this case also - of language and abstract/symbolic processing, and is systematically bound to consciousness. In humans, it may be related to the huge development of the cerebral neocortex.
The processing methods involved and the results produced can usually be verbally expressed, thus assuring an efficient communication between individuals and the development of a shared bulk of knowledge. There is no association with qualia. Functionality may be improved by adopting procedures of planned experience, and by utilizing purposely built external instrumentation.

The two pathways are not independent of each other.
The results yielded by the first one may pass under the scrutiny of the second. Corrections may be produced and fed back, and sometimes incorporated in a manner akin to habituation. Conversely, the second pathway relies extensively upon the first, often delegating the processing systems of this latter, and obtaining ready-to-use, though poorly accounted for, results (as occurs in scientific insight, for instance). This indicates that objective knowledge, too, besides its own, endures limits and biases affecting the subjective one, although in ways harder to detect.