Is there a way to add a front shock/suspension to my Retrospec Valen Rev bike?
Looks like the bike is one of these https://retrospec.com/products/valen-rev-electric-bike
In this case, it is a 1/18" steerer with a 20" x 4" wheel. Cheap shocks for this are readily available, not sure about quality shocks. Short answer is therefore Yes, you can.
Long answer sits around if you should. What do you expect to gain from shocks?You will upset the bike geometry (crown to axle distance has to increase to accommodate shock movement.) to some degree, you would also likely need a new stem and handlebars. Sorting out the reach and stem height might not be straight forward.
Overall, it would be expensive, and a lot of work, probably for little performance improvement. The whole idea of 4" tires is to remove the need for shocks as the tire will do the work, you would need to go to 100mm or more shock before any real benefit kicked in, and realistically, on such a bike, it would be a very skilled or keen rider to make use of that much travel.
A good justification to do this are things like "because you can" and "it will look better".