Hey! Today I was trying to explain to a friend why Johnlock will 99.9% definitely become canon in season 4/5 but I couldn't think of any concrete evidence to give them. Please could you give a couple of solid reason why they'd definitely become canon?

Hi Nonny! I’ve written a TONNE of Masterposts about this! Here’s a copy-paste from this post here!

The Johnlock is Already Blatant - TAB has made that extremely clear to me.
Softly Softly: The BBC’s Study on LGBT Relationships (2009) - Before Sherlock, the BBC was looking for a show to help represent LGBT relationships. A year later,Sherlock was commissioned. TJLC: Your Guide to Romance & Holmesian Deduction in the Sherlock Fandom - An interesting long post about many of the more blatant signs of Johnlock through the first three seasons. I look forward to when this post is updated for S4. Fave post, great read!
TJLC for the Uninitiated - A wonderful summation of all the tropes and clues in Sherlock so far that point to a Johnlock Endgame. Basically, if you have a friend that needs convincing, send them here. What Happens When the Writers Stop Joking About Sherlock and John Being a Couple - One of the reasons I believe in TJLC so wholeheartedly is because THIS has already actually begun. HLV has already started the “canon Johnlock” arc. The Unabridged Dictionary of Johnlock Tropes: 157 Romantic Tropes Used in the Gayest Show on TV - ifyouhaveenoughnerve goes through the 157 romantic tropes and uses actual examples from Sherlock for EVERY ONE OF THEM to support Johnlock. It’s long but TOTALLY worth the read.
Heteronormativity Caught You in a Haste? - A lovely list that gives you rebuttals for all the common arguments against Johnlock.
TJLC: The Show Doesn’t Make Sense Without it - The show has gotten to the point where, if it’s not real, it doesn’t make any sense; there is no plot without it.
Sherlock & Romance, AKA Why So Many People Ship Johnlock - A CLASSIC by@wsswatson, this is a GREAT read. It takes you step by step through the romantic arc that Sherlock portrays season-by-season. Definitely worth a read, if you still need some convincing. My Masterpost’s Johnlock Meta Section - Like it says on the tin, I also have my own extensively organized Masterpost which contains an extensive and never completed TJLC / Johnlock section. Blog Tag: Sherlock and John’s Relationship Arc - Because I love precise tags. As a heads up, I’m VERY meticulous with my tagging system, so you may end up in a loop on my blog. It’s my way of ensuring I’m like Wikipedia; once you start here, you NEVER LEAVE *evil laugh*@softsons TJLC Explained Video Series - Great series for new and old fans of the series! Goes through the tropes and subtext in the episodes! Currently ongoing, so be sure to subscribe.

Plus: 

Check out this post here, as it discusses why John will not turn down Sherlock when he confesses
And this post discusses WHY we had to use subtext to build up the arc

I hope this has helped you out! Good luck!