Can I use my Apple Watch Series 4 for 5 hours without charging?

they say 18 hours batt time

Apple Watch - Battery - Apple

so unless your apple watch 4g have battery with a rather low battery health then 5 hours should prove not significant problem

you check the battery health in

settings->battery->battery health

they advice people to have the battery replace when it's health goes under 80%

Apple Watch Service and Repair - Apple Support

If you need more time before you can get to a charger then

Press the side button and tap the battery % icon then select low power mode.