Can I activate Jhin Legend later or does it have to be instant. Say I have 4 banished and I want to use other things 1st So I can get the full amount from it can I activate it later? Or does it trigger a chain

No — Virtuoso isn't an activated ability, so there's nothing you can "activate later." It's a triggered ability (Rule 383): it fires on its own when you play a spell and actually spent [4]+ energy, and it goes onto the Chain like any trigger.

Your only choice in it is whether to banish the spell, and that "may" is decided when the trigger finalizes onto the Chain (Rule 383.3.a) — you can't hold the trigger in your pocket and fire it at a better moment later in the turn. Because it does open a Chain, you can play other things while it's sitting there (you have a Reaction window before it resolves), but the payoff clause — "Then, if there are four spells banished with me, put each in its trash, channel 4 runes, and draw 1" — is checked on resolution, not when it triggers.

Also worth noting: "when you play a spell" in that condition means the spell has resolved, so it fires after the spell is done, not while it's on the Chain (FAQ #8769). And since the four-spell payoff resolves immediately on the 4th banish, you can't bank four spells with him and cash them in later — the channel/draw happens on that trigger's resolution.