When Jesus says "forever", it means that which, or rather Who will remain among the disciples will remain so just as He (qua the Son, who shares sovereignty with Father in a drastic difference from any of the even highest angelic hosts who are just servants /Hebrews 1:14/) remains forever (John 8:35) and just as His Father together with Him (John 14:23). The power of "forever" applies only to the Three, for "forever" puts Them beyond anything that can apply to creation: frailty (for even angels can - and did - fall), temporality, unsteadiness, contingency.
Thus, when Jesus says "another Comforter" ("Encourager", "Advocate", "Intercessor" whatever translation you may offer for the Παράκλητος, but "Comforter" is warmer and better), He implies Himself as the correlate for the "another", for He was comforting His disciples until He was with them, but now He is to go to Father; thus, as He was a Comforter for His disciples while being with them, so will be "the Spirit of Truth who proceeds from the Father" (John 14:16), asserting thus the Spirit's equality with Him, and given that He asserts His own equality with the Father (John 5:23), thus He also asserts the Spirit's equality with the Father. There can be nothing else and other to whom the "forever" can apply with the same power as this term applies to the Three.
As to your question of "quadrinity", the difficulty is solved by the traditional explanation that Holy Spirit is one, but the intensity of His presence with humans can be very different. Indeed, Prophets also had the presence of the Spirit, for the entire Scripture is inspired by the Holy Spirit (2 Tim. 3:16; 2 Peter 1:21), however they did not have that baptism in Holy Spirit (Matt 3:11), that is to say, that intensity of presence and inner transformative action of the same Spirit, that only Jesus' advent could bring to humans, granting them authority to be called "sons of God" (John 1:12) and, accordingly, call God - "Father" (Romans 8:15). To give an analogy: one thing is when a violin teacher teaches you how to store a violin so as not to damage it; how to hold it correctly; how to even tune the strings (cf. Old Testament presence of the Spirit); but another thing is to teach how to read scores and actually play Mozart or Mendelssohn, or even venture and play one's own creative compositions (cf. New Testament-esque intensity of presence of the Spirit's inner action in the baptized).