Can 4-1" Innerducts fit in a 4" Conduit?

4 - 1" Outside Diam in a square configuration should fit in a (1+1.4142) ± 2.5" Inside Diam. with ease.

What you should do is measure the OD of the smaller and the ID of the bigger.

ID required = OD + sqrt (2 x OD) (allow for a loose fit)

please change to:

ID req. = OD + sqrt ( 2 x OD^2)

I DO IT EVERYDAY....WE USE 3X 1.5" INNDERDUCT + 1 X 1" INNDERDUCT FOR OUR FIBER OPTIC CABLES INSIDE THE PVC CONDUIT WE USE IN OUR CAMPUS COMMUNICATIONS SYSTEM. WE USE THEM T PROVIDE PROTECTION TO THE CABLES DURING INSTALLATION. . WE ALSO USE THE INNDERDUCT ON CABLE TRAYS TO PROVIDE A BARRIER TO FROM THE OTHER CABLES ETC ETC.........CHEERS

...guessing your objective is to tunnel just one conduit under a path, lane, road, etc., and then feed smaller conduits through the large...to save effort and ensure the pipes escape out the far end in like proximity as they went in--For DIYer, tunneled pipe will tend to stray off course en route to its destination. Although such an approach is sensible in concept, it can also be problematic. Consider some points:

• Even if the containing pipe can be pushed in with little or no surrounding voids (with almost original soil compaction around it) there will be unsupported voids (elastic spaces) within the containing pipe and around the contained pipes.
• This will render the containing pipe less resistant to flexion under load above. (This should enter into the decision as to how far below grade to tunnel.)
• As such it might be necessary to go with a higher schedule or different material for the containing pipe...but larger schedule (greater wall thickness) would decrease or eliminate clearance for the contained pipes. However...
• a larger-dia containing pipe could be chosen, but that could increase unsupported void inside the containing pipe...but possibly...
• It might be workable to inject expanding, hardening foam inside the containing pipe to fill the voids and further support the containing pipe (as well as contained pipes), to resist flexion under dynamic loading (vehicle traffic for example) from above... but
• the larger pipe will be more difficult to tunnel--and more bore or water bore removal of soil to allow the pipe to go through, probably resulting in more voids around the containing pipe--less support to dynamic loads overhead--more chance of subsidence (and coincidental cracking of any overhead paving)...But...
• To minimize or avert excess soil removal, pushing a large pipe can be done if left open at both ends--so it can "cut its way" through (a bore path less than its own diameter), after which it can be flushed. (The small pipes would be much more difficult to clear or flush out, so pushing them through a large pipe will be greatly advantageous.)
• The smaller pipes must also be longer in length than the containing pipes; so that any and all joints in the contained pipes will not be situated inside the containing pipe, the increase of diameter of the joints thereby decreasing or eliminating clearance in the containing pipe for for their passage....
• The most readily available pipes, all dia's, come in standard, 10-ft lengths...but writer has seen smaller PVC (conduit, but not sure about electrical conduit) offered in 20-ft lengths at Lowes.

Apologies if this is not your situation; perhaps it will help anyway?

This is a standard configuration that is generally used in construction.

4-1" innerducts or 3 - 1 1/4" innerducts are the norm.

If you are working on an underground installation, some companies like Carlon makes 4" PVC ducts with the innerducts already installed like the configurations listed above. This will save time as all you have to do is connect the ends of the conduit(s) and the innerducts will be automatically connected. You don't have to pull in the innerducts after the conduits are in the ground.

This should save time and money...