3rd density Earth will remain as it is, right?

[anagogy]

So what defines the 4th density? What are the limitations of it compared to 5th density? I know it's about love but how does it encourage the practice of love?

Just as the requirement for 3rd density incarnation is self-awareness, the requirement for 4th density incarnation is an open or crystallized heart center.

3rd density is the plane where positively polarized beings learn how to open themselves to universal compassion. 4th density beings have learned this lesson, and now seek the ways of wisdom.

This is what Ra has said about 4th density:

16.50 Questioner: Thank you. Is it possible for you to give a small description of the conditions [in] fourth density?

Ra: I am Ra. We ask you to consider as we speak that there are no words for positively describing fourth density. We can only explain what is not and approximate what is. Beyond fourth density our ability grows more limited still until we become without words.

That which fourth density is not: it is not of words, unless chosen. It is not of heavy chemical vehicles for body complex activities. It is not of disharmony within self. It is not of disharmony within peoples. It is not within limits of possibility to cause disharmony in any way.

Approximations of positive statements: it is a plane of a type of bipedal vehicle which is much denser and more full of life; it is a plane wherein one is aware of the thoughts of other-selves; it is a plane where one is aware of the vibrations of other-selves; it is a plane of compassion and understanding of the sorrows of third density; it is a plane striving towards wisdom or light; it is a plane wherein individual differences are pronounced although automatically harmonized by group consensus.

4th density is less physical than 3rd density. 5th density is less physical than 4th density. As you go up in the densities, the time/space component grows larger while the space/time component becomes smaller.

Essentially, all densities are vibrational spectrums of conscious expression. The higher densities are more dense with consciousness, but less dense with matter. It's difficult to properly assess the limitations of each density beyond the one we currently inhabit. All we can really do is speculate based on what we have researched through our inner work and dialogues with higher density beings such as Ra.