Start with the same updated game state
Have every player finish updates, close BL4 completely, and relaunch before forming the party. The official update notes explicitly ask players to restart so updates apply and matchmaking connection errors are avoided.
Write down whether joining fails, a loaded session disconnects, or the host’s game crashes. Those are different problems and require different evidence.
A short test matrix
Use the same two players and repeat a simple activity that both can access:
| Test | What it tells you |
|---|---|
| A hosts, B joins | Establishes a baseline combination |
| B hosts, A joins | Tests whether the symptom follows the host |
| Both play solo | Separates a local crash from online-session trouble |
| Same party in base-game content | Helps isolate a DLC entitlement or mission-specific issue |
| Same party on another connection you control | Helps isolate the local network without changing router rules |
Record platform/store, game version, time, mission and exact error for each result. Changing the host is a comparison, not a claim that the game automatically migrates hosts in every failure case.
If joining fails immediately
- Confirm both players are signed into their platform and the game’s online services.
- Check platform/service notices and whether other friends can join.
- Use a fresh invitation after both players restart; avoid repeatedly retrying a stale invitation.
- If the message concerns content ownership, verify the required DLC entitlement on each platform before changing network settings.
Do not leave a Steam Family, unlink an account, or forward a broad port range based only on one failed invitation. This page has no verified BL4-specific port-forwarding or steam://friends/refresh fix.
If the session loads and then drops
Record the event immediately before failure: fast travel, a mission hand-in, a particular boss, opening a menu, or an ordinary idle period. Try the same event once with the other host. A repeatable mission trigger is valuable evidence for support even when a home connection otherwise works.
If the host application closes, use the startup/crash diagnostic and include the crash time. If characters remain connected but movement is delayed, record that separately from the game’s connection error.
If the symptom follows one network
Restart the router using its normal procedure and compare wired Ethernet to Wi-Fi if available. Pause large downloads during the test. Use the network diagnostics supplied by your platform or ISP and keep the results.
Router changes depend on the actual network, including managed networks and carrier-grade NAT. Ask the network owner or ISP about an identified restriction; do not enable UPnP or expose ports just to try every possible setting. A VPN is another variable, not a guaranteed co-op repair.
FAQ
Only one friend fails to connect. Is it definitely NAT? No. The host matrix, platform login, version and content entitlement checks help narrow it down. A failed invitation alone does not prove a NAT mismatch.
Should we restart after a weekly update? Yes. This is an explicit instruction in the official update notes, even when the update is server-side activity content.
Can Steam Remote Play replace online co-op? This guide does not establish BL4 support for that feature. Use the game’s documented supported multiplayer flow.
Source and reporting a remaining problem
This checklist follows the official PC troubleshooting guide, checked October 18, 2026. It is a diagnostic sequence, not a promise that one step fixes every PC. Record your CPU, GPU, driver, resolution, game version, exact error and the result of each test. Send those details with DxDiag and MSinfo to Borderlands Support if the problem remains.