What follows is the text of the scanned pages that links to were posted by aglorincz above. If Nikiphoros the Monk is the same as Nicephorus the Solitary, and the Philokalia that has been provided for us in translation (link up-thread), then I believe we should conclude that there is no missing 5th volume, unless there is evidence to the contrary.
From Nikiphoros Himself
And if the Most High is not your refuge you will not lay up His love in your heart.
Most if not all of those who attain this greatest of gifts do so chiefly through being taught. To be sure, a few without being taught receive it directly from God through the ardor of their endeavor and the fervor of their faith; but what is rare does not constitute the norm. That is why we should search for an unerring guide, so that under his instruction we may learn how to deal with the shortcomings and exaggerations suggested to us by the devil whenever we deviate left or right from the axis of attentiveness. Since such a guide will himself have been tested through what he has suffered, he will be able to make these things clear to us and will unambiguously disclose the spiritual path to us so that we can follow it easily. If you have no such guide you must diligently search for one. If, however, no guide is to be found, you must renounce worldly attachments, call on God with a contrite spirit and with tears, and do what I tell you.
You know that what we breathe is air. When we exhale it, it is for the heart's sake, for the heart is the source of life and warmth for the body. The heart draws towards itself the air inhaled when breathing, so that by discharging some of its heat when the air is exhaled it may maintain an even temperature. The cause of this process or, rather, its agent, are the lungs. The Creator has made these capable of expanding and contracting, like bellows, so that they can easily draw in and expel their contents. Thus, by taking in coolness and expelling heat through breathing, the heart performs unobstructed the function for which it was created, that of maintaining life.
Seat yourself, then, concentrate your intellect, and lead it into the respiratory passage through which your breath passes into your heart. Put pressure on your intellect and compel it to descend with your inhaled breath into your heart. Once it has entered there, what follows will be neither dismal nor glum. Just as a man, after being far away from home, on his return is overjoyed at being with his wife and children again, so the intellect, once it is united with the soul, is filled with indescribable delight.
Therefore, brother, train your intellect not to leave your heart quickly, for at first it is strongly disinclined to remain constrained and circumscribed in this way. But once it becomes accustomed to remaining there, it can no longer bear to be outside the heart. For the kingdom of heaven is within us (cf. Luke 17:21); and when the intellect concentrates its attention in the heart and through pure prayer searches there for the kingdom of heaven, all external things become abominable and hateful to it. If, then, after your first attempts you enter through your intellect into the abode of the heart in the way that I have explained, give thanks and glory to God, and exult in Him. Continually persevere in this practice and it will teach you what you do not know.
Moreover, when your intellect is firmly established in your heart, it must not remain there silent and idle; it should constantly repeat and meditate on the prayer, 'Lord Jesus Christ, Son of God, have mercy on me', and should never stop doing this. For this prayer protects the intellect from distraction, renders it impregnable to diabolic attacks, and
everyday increases its love and desire for God.
If, however, in spite of all your efforts you are not able to enter the realms of the heart in the way I have enjoined, do what I now tell you and with God's help you will find what you seek. You know that everyone's discursive faculty is centered in his breast; for when our lips are silent we speak and deliberate and formulate prayers, psalms and other things in our breast. Banish, then, all thoughts from this faculty - and you can do this if you want to - and in their place put the prayer, 'Lord Jesus Christ, Son of God, have mercy on me', and compel it to repeat this prayer ceaselessly. If you continue to do this for some time, it will assuredly open for you the entrance to your heart in the way we have explained, and as we ourselves know from experience.
Then, along with the attentiveness you have so wished for, the whole choir of the virtues - love, joy, peace and the others (cf. Gal. 5:22) - will come to you. Through the virtues all your petitions will be answered in Christ Jesus our Lord, to whom with the Father and the Holy Spirit be glory, power, honor and worship now and always and throughout the ages. Amen.
I wish to point out, that while prayers and mantras, have been an aid to contemplatives as far back has history, and even the archaeological record, goes it is my finding that once you can still your mind, and keep it so, then do not bother with prayers, mantras, meditation aids or techniques, but just abide in that silence ever vigilant for the charisms that will indeed arise. And, when those charisms arise know that that is the Christ within that the Christian mystics of the past spoke of; and it is one and the same as the Yoga, of the yogis in samadhi.
Link to the original
Greek PhilokaliaAnother Link to the original
Greek PhilokaliaYet, another Link to the original
Greek Philokalia