I have a 420 small block that when ever it sits over night or longer it takes 15 to 30 seconds to register oil press.The line that goes to the guage is AN4.Using Wix 51515r filter.Any ideas guys?

Are you using a remote filter on it... if so .. its
draining back down

air in the psi guage line might be some of the prob.mine took five seconds to get full oil pressure acording to the guage in my boat, the Racepak told a much different story. (one point six seconds)

another reason to use an accumulator

does the engine have a lot of miles?
if it's an old oil pump, you might want to pull it. pull the oil pan as well and make sure the pickup is not half plugged up with gump.

if it's a fairly new engine, you should still check out the oil pump. sometimes the spring in the pump can be defective, or not seated right etc.

better safe than sorry.

IMO it's because of the #4 line, you would have quicker response with #3 line, no worries.