The thermostat in a 1999 Jeep Wrangler 4.0L typically opens around 195-205°F (90-96°C). The engine will operate *slightly* above and below this temperature, fluctuating based on load, ambient temperature, and the efficiency of the cooling system.
While a few degrees above or below the thermostat's opening temperature is normal, consistently running significantly hotter or colder could indicate a problem. If the engine is overheating (significantly above 220°F/104°C) or running noticeably cool, there's likely an issue requiring attention.
In short: Aim for normal operating temperature around 195-205°F (90-96°C), with minor fluctuations acceptable. Anything significantly outside that range warrants investigation.
