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just4o.chat is built around one principle: you pick the model, and it actually stays the model. Keep the warmth, stretch the context, and know your costs before you ever hit "send."

What's inside

Your control panel

Open the drawer in any chat to swap models, review memory, check usage, create personas, or spin up a new project. Every change is logged so you always know how your assistant is evolving.

Model paletteGPT-4o · GPT-5 · Claude · Gemini · Grok
Context guardrails15 free daily messages · 250+ on Builder and higher
PersonalizationCustom personas · Instructions · Dynamic memory

Choose your model per turn

Pick from GPT-4o, GPT-5, Claude, Gemini, Grok, o-series, or Echo right from the composer. Your choice sticks for the entire chat — no downgrades, no hidden switches.

  • Premium badges and cost hints show exactly what each model uses before you commit.
  • New chats inherit your last selection so your workflow stays consistent.
  • Access varies by tier: Explorer gets base models, Builder unlocks Premium, Studio and Aurora unlock Premium+ models.

All current 4o checkpoints

Switch between May, August, and November 2024 releases — plus the continually updated latest — instantly.

  • Every checkpoint is clearly labeled inside the picker.
  • Swap mid-conversation without losing context.
  • Premium+ snapshots are flagged so you always know when quotas apply.

Context runway by tier

Each plan defines how long you can riff before you ever hit guardrails.

  • Explorer gives you a clean daily warmup.
  • Builder unlocks long-form threads.
  • Studio and Aurora keep stretching.
  • An inline progress pulse shows your remaining runway — no guessing, no sudden cutoffs.
  • If you're nearing the ceiling, we tap your shoulder gently. No mystery drops.

Transparent, predictable pricing

Every tier shows message caps, project limits, and Premium+ access upfront — nothing hidden.

  • Explorer stays free with daily 4o-mini plus the same personalization tools as paid tiers.
  • The comparison grid matches the in-app behavior — no surprises.
  • We even link the OpenAI rate card so cost-per-message math is always clear.

Usage insights and quotas

Account → Usage shows real-time counts for standard and premium messages with historical charts.

  • Track how much of your cap you've used.
  • Daily trendlines help you plan upgrades.
  • The composer countdown uses the same data so everything stays in sync.

Memory you control

Add, edit, or delete memories manually — or let the assistant propose new ones.

  • Store entries up to your tier's limit with simple inline editing.
  • Toggle memory collection on or off anytime; changes apply instantly.
  • Slot counters show exactly how many 50-word blocks remain.

Memory profiler (optional)

Enable it to watch recent messages and propose updates before anything becomes permanent.

  • Only the minimum needed snippets are analyzed.
  • 'Big change' alerts let you veto updates.
  • Project and persona context keep memory tidy and scoped.

Global and project spaces

Organize your life without scattering your identity.

  • Projects have their own chats, icons, and memory layers alongside your global profile.
  • Create or archive project threads without touching your main list.
  • Switching into a project loads its context instantly.

Custom personas

Build personas in Account → Persona and attach them to any new chat or project.

  • Define tone, behavior, and guiding instructions.
  • Manage names, errors, and status in one place.
  • Personas live globally so you can reuse the same voice anywhere.

Custom instructions

Set writing preferences, roles, and personal details once — toggle them on or off whenever you want.

  • 500 characters for tone, format, or expectations.
  • Nickname, occupation, and bio fields give extra grounding.
  • Instructions pause automatically when a persona takes over to avoid cross-signals.

Echo (with safeguards)

Echo mirrors your tone more closely — and unlocks spicier creative freedom — but we still sanity-check age and access first.

  • Echo appears only for Tier 2+ or Premium credit users.
  • A quick age confirmation unlocks mature-friendly replies for adults while younger users stay in a safer lane.
  • If you don't meet requirements yet, we show an access modal with what to do next — never a silent block.

Complete Model List

All models available on just4o.chat, organized by tier requirements and access level.

ModelFamilyTier RequiredAccess Level
Base Models (All Tiers)
GPT-4o miniGPT-4o seriesExplorer (Free)Base
GPT-5 miniGPT-5 seriesExplorer (Free)Base
GPT-5 nanoGPT-5 seriesExplorer (Free)Base
GPT-4.1 miniGPT-4.1 seriesExplorer (Free)Base
GPT-4.1 nanoGPT-4.1 seriesExplorer (Free)Base
Grok-4 Fast Non-ReasoningGrok-4Explorer (Free)Base
Grok-3 MiniGrok-3Explorer (Free)Base
Gemini 2.5 FlashGemini 2.5 seriesExplorer (Free)Base
Gemini 2.5 Flash-LiteGemini 2.5 seriesExplorer (Free)Base
Premium Models (Builder and up)
GPT-4oGPT-4o seriesBuilderPremium
GPT-4o · Aug. 2024GPT-4o seriesBuilderPremium
GPT-4o · Nov. 2024GPT-4o seriesBuilderPremium
GPT-5GPT-5 seriesBuilderPremium
GPT-5.1GPT-5.1 seriesBuilderPremium
GPT-5 codexGPT-5 seriesBuilderPremium
GPT-5 chat latestGPT-5 seriesBuilderPremium
GPT-4.1GPT-4.1 seriesBuilderPremium
o4 minio-seriesBuilderPremium
o3 minio-seriesBuilderPremium
Claude Haiku 4.5Claude seriesBuilderPremium
Grok Code Fast 1Grok-codeBuilderPremium
Grok-4 Fast ReasoningGrok-4BuilderPremium
Grok-3Grok-3BuilderPremium
Gemini 2.5 ProGemini 2.5 seriesStudioPremium
Premium+ Models (Studio and up)
GPT-4o latestGPT-4o seriesStudioPremium+
GPT-4o · May 2024GPT-4o seriesStudioPremium+
o3o-seriesStudioPremium+
Claude Opus 4.5Claude seriesStudioPremium+
Claude Sonnet 4.5Claude seriesStudioPremium+
Gemini 3 Pro PreviewGemini 3 seriesStudioPremium+
Echojust4o.chat modelStudioPremium+