just a few tips for rendering archi stuff in cinema. i got loads more, and if you have any, then just keep adding to this post :
Radiosity/gi - Radiosity engine in C4 is the fastest currently on the market, but obviously the higher the settings the longer the render times. For arch rendering never use a diffuse depth of greater than 1. this is the natural setting. Always use as higher accuracy as possible. personally i use 100%, but if ur pc no that good then anything over 75% should suffice. pre-pass size - i use 2 for final rendering. Stochastic samples - atleast 300. i use 500+ (even that sometimes shows artifacts). Min. samples - a value of 20-30 should be optimal for most scenes. max samples - high as poss. i use 100+. For better radiosity rendering us stochastic mode, again samples to 300+ and diffuse to 1. Stochmode takes longer to render but gives a true, more accurate render.
When set the rendering in motion close down the scene from the mem buffer. just saves extra speed.
the Radiosity strength should ALWAYS be set at 100%. to adjust the strength use either render tags or illumination settings in the material editor. render tags can force objects to render with radiosity or not, at whatever percentage you choose. the illumination controls in the mat. editor controls individual material gi settings.
And remember, things that dont need gi too much, like tree leaves, scattered grass etc.. turn the gi transmit button to OFF, else long un-needed render times will be there.
Stick to normal simple materials if you can. proceedurals and SLA take allot more processing than normal materials.
minimise the render window when rendering and dont continue editing or multi tasking durring rendering. all slows the process down.
dont bother trying caustics - in the main archi renders dont use caustics. not needed and not noticed.
HDRI - personally i think this is just a tarty buzz word ppl are using and imho definately not needed. anyway, so easy to fake.
But my main tip (as i stated) is the render tags. - render tags are found by right-clicking on an object. these control display properties of individual objects. GI levels, raytraced levels etc... are controlled here and a shed load of render time can be saved using these render tags.
enough for now, but i'll add to the list when i can think of some more.