Peaceable Projects Inc. is a U.S.-based non-profit group that brings the vast resources of the wide world together with the ongoing needs of the people who live, work, and travel on the Camino de Santiago pilgrim trail network in Spain.
The weather is almost impossible to predict. I walked from Porto in early April 2017 and it was a heat wave. High 20's C, even low 30's C some days. Walked to Santiago and didn't have a single drop of rain the entire way. The only rain we encountered was on our bus trip back to Madrid.
Normally April can be wet and cool in Portugal and Spain. But what is normal anymore?
It really comes down to the luck of the draw for weather.
Be prepared for any weather possibly and hope for the best. Camino's are still awesome rain or shine, so don't get stressed over something you have absolutely no control over.
On the plus side it will not be as busy in April as it will by mid May when Camino 'prime time' starts.
The cathedral (Se) in Porto do normally have credentials sold in the little shop - but as far as i remember the cathedral is only open certain hours and is closed at lunch time. Depending on your schedule it may be a good idea to get one in advance eg from the Forum shop or a local confraternity.
The weather in spring can be very changeable - but given you are walking in an area more defined by atlantic weather it is likely you will get rain and so be prepared! I walked Coimbra to Vila do Conde in April and got a mix of weather - heavy rain, sun, fog, showers, wind but walked Vila do Conde to SdeC in Janaury and had blue skies every day - frost in the morning but sat out for coffee by lunchtime. So layers are the way to go! (and bring suncream too!)
Camino Weather Map helps pilgrims plan their trip according to the weather. Camino Weather Map outlines the main Camino routes and their weather per month
The principal to plan for are NOT the extremes, but for the averages in temperatures, both highs and lows. A weather shell is always necessary to carry, and will serve for both rain protection and as a protection from wind chill.
One option for your Credencial is to purchase one from Ivar's store (Ivar is the Forum's owner, so purchases help the Forum)
The photo above is of a passport with stamps. This passport comes with no stamps, you collect them when you walk. ⚠️ Note: I also sell the HOLY YEAR Credential if you prefer this one. Both these credentials are valid during the holy years and also the years after. Note that this passport...
In Porto, you can pick one up at the Sé Cathedral. I am not sure what hours they are open to do so.